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I have a P3 700 MHz cpu, and a GeForce MX4000 card.. I get about 2fps in Painkiller. What fps would I approximately get if I had a P4 4 GHz cpu ?
Link b/w processors and gfx cards
FPS depends upon lots of factors . CPU , GPU , Harddisk Speed , Ram ...Motherboard used ..
Ok your present Configuration is p 3 700 Mhz so what one can guess is this .....
1) You have a PPGA socket .
2) You have 810 or 815 Based Board.
3) You are using SD Ram
4) AGP 4X ( Not that it will matter much here )
Now all this gives you 2FPS in Pain Killer ( Wonder how you got this low FPS and were able to actually count it )
What P4 4 Ghz ( 4 Ghz you said ....hmm now down to eath take that 3 or 2.8 ghz ) will do for you is here :
1) You will have P 4 with Higher FSB
2) An 865 MOBO With higher FSB
3) You will have a DDR Ram..( May be 256 MB )
4) AGP 8X ( Again 4X and 8X is not a difference worth nothing )
Considering the GPU or Graphic card is same You will have hmm far as i can imagine and guess ( Guess because there is no known benchmark to gauge that )
You will have 28 to 35 FPS depending on your setting .
What this means in a nutshell is 25 FPS is playable ..and good for human eye. So yes an upgrade will help you
Luck
Ok your present Configuration is p 3 700 Mhz so what one can guess is this .....
1) You have a PPGA socket .
2) You have 810 or 815 Based Board.
3) You are using SD Ram
4) AGP 4X ( Not that it will matter much here )
Now all this gives you 2FPS in Pain Killer ( Wonder how you got this low FPS and were able to actually count it )
What P4 4 Ghz ( 4 Ghz you said ....hmm now down to eath take that 3 or 2.8 ghz ) will do for you is here :
1) You will have P 4 with Higher FSB
2) An 865 MOBO With higher FSB
3) You will have a DDR Ram..( May be 256 MB )
4) AGP 8X ( Again 4X and 8X is not a difference worth nothing )
Considering the GPU or Graphic card is same You will have hmm far as i can imagine and guess ( Guess because there is no known benchmark to gauge that )
You will have 28 to 35 FPS depending on your setting .
What this means in a nutshell is 25 FPS is playable ..and good for human eye. So yes an upgrade will help you
Luck
Well mate the link between the Processor and the GFX Card is not the only thing responsible for the frame rates. The Total RAM and its Speed, the OS version, the Graphics drivers, all these do play there part in the overall fps. Of course the faster the CPU the faster the fps. But then Just having the fastest processor with a poor GFX card or the best GFX card with a poor CPU are just as bad. All the components should sync with each other too produce a decent gaming rig. So as freshseasons said upgrading would definately help.
For a Start Ur P3 is The Most Biggest Bottleneck 
.....Dun expect These games (Including Doom 3) which bring Even The Best Processors Including The P 4 to its knees To give Frame Rates At ur current config.......Say U get a P4 2.8 Gigz Then too u will need the best of the Graphic card and even Physical Memory (Atleast 512 To get Things goin smoothly) to get Ur game up and runnin.....If Say U get it up with a Ati Radeon 9800Pro And The Same for Geforce's Best 6600 Ultra.....Then U can really enjoy these games at Their true Fps Rate ie anything From 50 to o 80 (Or may be higher Depending on the game Not so sure) at normal resolutions and may be somewhere between 60 to 90 Fps at all the eye candy high Resolution settings.......This is also considering the Game that u are playing as The fps that u would be getting For Doom 3 Wont be the Same as On Half Life 2,Pain Killer,Far Cry And So On..... 
.....Dun expect These games (Including Doom 3) which bring Even The Best Processors Including The P 4 to its knees To give Frame Rates At ur current config.......Say U get a P4 2.8 Gigz Then too u will need the best of the Graphic card and even Physical Memory (Atleast 512 To get Things goin smoothly) to get Ur game up and runnin.....If Say U get it up with a Ati Radeon 9800Pro And The Same for Geforce's Best 6600 Ultra.....Then U can really enjoy these games at Their true Fps Rate ie anything From 50 to o 80 (Or may be higher Depending on the game Not so sure) at normal resolutions and may be somewhere between 60 to 90 Fps at all the eye candy high Resolution settings.......This is also considering the Game that u are playing as The fps that u would be getting For Doom 3 Wont be the Same as On Half Life 2,Pain Killer,Far Cry And So On.....