Re: First build...
Ok, first thing, that GPU is very dated. Its very confusing, the GTS 320 and 640 are the old GTS version (G80), the new one is the GTS 512 (G92), is just another level above those two. A rough guide to graphics card performance, in increasing order of performance:
So go GTS 512 instead in my view. Certainly don't get the 640 SSC.
Right, thermal paste... erm yes, If you can, get some aftermarket stuff. In order of preference, best first:
Arctic Silver 5
All will do a great job, MX-2 is prolly the pick of the bunch. Its worth spending the extra $5 or so on a good paste. Given how much you are spending on the HSF.
EDIT: Oh yes, maybe get a Seagate Barracuda 7200.10 or 7200.11 model over that WD caviar.