All the games made past 2006 are gpu bound at higher resolution with some exception....it will not matter much at higher resolution....and for the e-series c2d its not that the game will bring down pc to its knees.....its only when u play instead of 60+ fps on a full blown config u will get 35~55 fps on a particular resolution.
cpu only bottlenecks when resolution used upto 1280*1024 where cpu & gpu are on equal platform...above that its gpu on the front and cpu to the backing.
(all AI,scan matrix calculation is done by cpu and all rendering is done by gpu.)
at 1080p all job goes to gpu (75:25 ratio gpu:cpu)
as for @OP's question go for hd6850 upgrade cpu later.....play at higher resolution more than 1280*1024 u will b safe but look for good amount of ram,better hdd,best psu.
And yes bottleneck will be there if u go on max setting on every game....dude try to play on custom setting most suited for your config.
Have fun.
hmmm i guess i will probably play most games at 1080p like i mentioned earlier coz im getting back to gaming after a hiatus of 4 years

and right now im using ddr2 2x1gb 800mz kingston ram which i will be upgrading to ddr3 when i upgrade my cpu and mobo . the ram will be fine for the moment right ?
btw i was thinking of buying the following along with the hd6850
Cabinet : NZXT Gamma
PSU : corsair cx450w
Monitor : Benq G2220HD
are the above mentioned components fine or do i need some changes there ?