8800 GTS 512 evga is a dandy i'm told.
E-vga has a step up program, so if you're not happy with the card you can mail it away for a trade in at a split difference cost as long as you keep your receipts.
I'm using 8800 GT SLI, I had started with a single 8800 GT and I was pretty happy with it but I upgrading to dual 8800 GT so I could play crysis with eye candy :P
Test your ram, its quite possible that is the problem. A lot of higher end boards you need to manually set your ram timings/voltages amongst other things. Make sure your bios has the same settings as what your ram manufacturer is supplying.
Not withstanding the fact that ram is a very sensitive component and one of the more common causes for system stability. Memtest is a good program~