Just get a low end $20-$30 GT430/GT440. This video shows a comparison and has some setup info specific to borderlands:
Borderlands 2 AMD Nvidia Hybrid PhysX - YouTube
basically install the 285 driver, install the standalone physx, run the extra hybridize tool. Rename/delete the physxcore.dll and physxdevice.dll files in the borderlands bin folder. You can use gpu-z to confirm physx is enabled on the ATI card and make sure it's put load on GPU2 during gameplay.
Obviously there is still a lot of testing to be done with Borderlands 2 as we plan to integrate in our GPU testing going forward. As we have done in the past, the chances are good that we will do so setting PhysX at the lowest possible setting - it annoys NVIDIA when we do that but it seems to be the most fair way to compare the performance of these varying GPUs. Especially until we figure out exactly WHY the performance is dropping so dramatically on AMD cards when PhysX is at Medium or High.
It is nice that least AMD Radeon users have the option to enable PhysX at all, and if you are playing with a high performance cards like the HD 7900s, you will probably still be able to set it to Medium and get playable frame rates at 1080p. No, NVIDIA hasn't enabled PhysX acceleration on AMD cards, but they appear to have chosen to allow it to run through the system as a whole (or maybe they just forgot).
UPDATE: I did finally get an answer from Gearbox about the slow downs we were seeing on the AMD results. Apparently when larger collections of PhysX simulations are running on the CPU, those threads can take quite a bit longer than they would when running on the GPU. As a result, the CPU (and rest of the game engine code) becomes "blocked" waiting for a single thread to finish, which results in the lower CPU utilization we saw on the AMD results as well as the lower overall performance. Because PhysX is an NVIDIA engine, even if Gearbox chose to they likely couldn't add in additional multi-threaded capabilities to the PhysX code path so the slow down here is likely to stay.
For most NVIDIA GPU users, the additional PhysX effects are a fun and simple way to increase the "cool" factor of your gaming experience and you should be able to run at Medium or High pretty easily. That is just one of the advantages you get with the NVIDIA GeForce brand with this title. Mid-range AMD users will want to leave PhysX at low for the best frame rates but if you have a high end system and utilize a higher end Radeon GPU you can probably still get away with the Medium setting.
Taken from:*www.thinkdigit.com/forum/graphic-cards/180508-dedicated-nvidia-gpu-physx-needed-when-using-r9-270x-main-gpu.html
[h=2]Borderlands 2: is CPU capable of handling the PhysX effects?[/h] with 25 comments
Borderlands 2, latest and probably one of the greatest games with support for GPU accelerated
PhysX effects, is a HOT topic these days.
Update: GPU PhysX in Borderlands 2 – PhysX review and comparison video
Update #2: Borderlands 2 PhysX Benchmark Roundup Usually, extra PhysX effects are meant to be executed on compatible NVIDIA GPUs, so even if one can force his CPU to do the work, it is not very effective – massive slowdowns and fps drops during scenes with intence physics are make the games hardly enjoyable. This is valid for titles like Batman series, Alice: Madness Returns, Mafia II and others. Said matter was a tough topic over recent years, even resulting in claims that NVIDIA “hobbles” the CPU PhysX performance by purpose, to make their GPUs look more advantageous. However, recently we saw many reports (mostly from AMD users) that
Borderlands 2 shows
surprisingly good performance, while running with all PhysX effects enabled even without a NVIDIA card in the system. Thus, we decied to perform a little investigation to answer the question –
can a CPU handle all the extra PhysX effects in Borderlands 2 ? A boss fight against “Boom and his brother Bewm” is a good candidate for PhysX testing – scene contains a
lot of particles (also, particles are constantly generated over time) and some
cloth objects. *physxinfo.com/news/wp-content/uploads/2012/09/border2_bench1_sm.jpgGun in our hands has explosive rounds - this produces even more particles
Testing system: i7 2600K CPU, GTX 580 GPU, 8 GB RAM, Win 7 64-bit. In-game settings: 1680×1024, all High. 60 sec sequence was recorded with FRAPS.
*physxinfo.com/news/wp-content/uploads/2012/09/border_bench2.png
First of all, Borderlands 2 is showing
excellent performance while utilizing GPU for PhysX calculations – minimum framerate is above 60 even on high PhysX settings. But most interesting, CPU execution can also provide
playble framerate, despite the fact that Borderlands 2 is still using good old PhysX SDK 2.8.4. We assume that some scenes in the game, containing lots of fluids, particle effects or your co-op friends, may result in lower fps (~15-20), but still – framerate won’t crawl most of the time, like in previous games.
UPDATE [26.09]: we have recieved several commentaries, that the scene we have choosen for our previous test, while being pretty heavy on PhysX effects, is still not producing too much load for the CPU. Thus, we decided to give it another try, but now using one of the most demanding levels of the game, as we heard –
Caustic Caverns. *physxinfo.com/news/wp-content/uploads/2012/09/border_bench3_sm.jpgCrystalisks are vomiting SPH Fluids
A place near the beginning of the level presents a good opportunity for testing, as it comprises several high-poly cloth pieces, many particle effects, Crystalisks and Varkids spitting SPH liquids. Results are averaged over the two runs for each of settings.
*physxinfo.com/news/wp-content/uploads/2012/09/border_bench4.png
While GPU performance took a large hit, it still can maintain minimum framerate above 30 fps and keep average framerate at pleasant 50 fps. CPU execution of PhysX effects, in comparison, crawls at nearly unplayable 15-20 fps level. We can now clearly say that
fluid simulation is affecting the CPU performance the most.
UPDATE #2 [28.09]: final round of testing, now using PhysX benchmark, built into Borderlands 2 (will be available soon in a patch).
It is designed to extensively utilize all kind of PhysX effects during the sequence.
*physxinfo.com/news/wp-content/uploads/2012/09/border_bench5.png
As you may see, numbers are located somewhere in between our two previous results – benchmark scene indeed includes some complex physics interactions, but the ones you may encounter in real game will affect your framerate in more dramatical way.
To summarize: If you are playing Borderlands 2 in Single Player mode, carefully avoiding fluid emitting weapons/enemies and staying away from certain areas of the game – you may find a CPU execution of PhysX effects sufficient. But if want really
comfortable gameplay, without any compromises –
presence of NVIDIA GPU is still a mandatory.
P.S. if PhysX option is grayed out on your system, you still can change the settings using "PhysXLevel" parameter in "WillowEngine.ini" file:
(\Documents\My Games\Borderlands 2\WillowGame\Config) PhysXLevel=0 - PhysX Low
PhysXLevel=1 - PhysX Medium
PhysXLevel=2 - PhysX High
P.P.S. Hybrid PhysX configurations are also supported.
PS: I got good PhysX rendered on my HD7770.This is why I asked whether a dedicated GPU is required or not.I installed just PhysX standalone drivers.
you mean gpu processor 
