You don't have a bottleneck in that CPU. It's about as good as a stock X6800. (Less cache but higher frequency, thanks to the overclocking.) Go for the 8800 GTX if you can afford it.
You may need a new PSU too, not just a new case.
Edit: if you don't have 2 GB of RAM already you should get to 2 GB, it would be a good investment.
Totally agree with the man. If you can't afford the 8800GTX, go for the GTS 640. You will see a big difference with that and 2 gigs of ram. You will need a good PSU, select from this list something with at least 32A on the 12V rails for a GTS, 36A for GTX.http://forumz.tomshardware.com/hardware/modules.php?name=Forums&file=viewtopic&all=1&t=222154&postdays=0&postorder=asc
The case doesn't have to be fancy, just be sure it fits the video card.