Re: Upgrades again...
the new ATI Radeon HD 4830 will perform just slightly better than the NV 9600/9800GT for the same price (around $120-130 i believe), and the 4850 will perform as well as the 9800GTX+, again around the same price (about $150-175)... what you go with really depends on your preference (i personally prefer ATI cards)
if you wanna OC that CPU, make sure its the E0 stepping version, and get a P45 based board... i'd suggest the BioStar TPower I45, Gigabyte EP45-DS3R/P or the Asus P5Q Pro/Deluxe (which you get depends on what features you want)... They'll get you some great performance outta that CPU (around 4GHz pretty easily)
if you go with an Intel chipset, i'd recommend an ATI card, because it will give you the option to setup CF later if you need to.