Well for a GTX yes it would be bottlenecked. A GTS would probably fit nicer in you system. Firingsquad has a good review on how a GTX can get bottlenecked on different processors.
All this being said I think you've hit your last leg with a single core processor. All big games that are being released this year are going to be multithreaded. Valve, ID, Crysis, Microsoft are all going to take advantage of 2 or more cores. You might want to find some sort of upgrade path too a multicore \ DX10 system and what I would suggest is start with your processor. Move your way up with that and let more DX10 cards come out.