It really depends on the game, however some games like farcry and any games running the source engine (hl2, counterstrike source, day of defeat source etc) will be quite bottlenecked by the processor in your case.
I had an athlon 1700+ up until recently and I had purchased a 6800gs to replace my aged 9700pro and was surprised to find that on games like counterstrike there was very little improvement in framerate, only when the game settings were completely turned up.
So i was pissed and returned the 6800gs and bought a 7800gt with a new athlon 3700+, problem solved.
I think if you're considering making a new purchase, then if you can only afford one or the other, get the 7800gs, then when you have the $ get a new generation processor to follow suit, you'll notice a difference when you change each piece depending on the game.
not really price efficient anymore and it doesn't run pixel shader 3.0 model or hdr, you'd be best off with a 6800gs or if you can't afford it the 6600gt (make sure its the gddr3 model tho, there are a few cheap-o's floating around).
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