You don't mention memory........hopefully you will be using 1GB (or more)
Otherwise good..........the X850XT doesn't support Pixel shader 3.0 so you wont be able to try the "next gen" effects such as HDR lighting.Don't let this put you off,I have an X800XT and that plays all the games I have well..including F.E.A.R. and OBLIVION.I don't know what price the card you are buying is but you could also consider an NVidia 7600GT which is one of the best "bang for buck" cards around right now (£140 ish).slightly faster than the card you have picked(unless you add antialiasing)and fully up to date feature wise.
Also for gaming,relatively few games are dual cpu optimised and you could get an Athlon 4000+ for a similar price to a 3800+.It is going to be faster on most games...dual core only really shows its muscle when multitasking.
It's well worth a look at the guides and reviews of cpus and graphics cards on click here