Don't bother buying a 9800GT that is just plain stupid, get the 260 GTS at least. If you're on a budget just get a damn 8800GT, because the 9 series is just a buffed up 8 series that is barely buffed up (i run a 8800gt, works perfectly fine on gta4, and all source)
As for a processor, I run a OC'd Q6600 at 3.2GHz (its awesome), but if you're only playing source save some $$$ and buy a core 2 duo instead of a quad, quads are a waste of cash unless you're compiling shit or hosting a server :/