Hi mate
Yeah, Intel are meant to be coming along with some new chips and some price cuts in june/july. New chip wise, the one Im keeping my eye on is the E6850 - $266 (so about £130-170), with a 3Ghz stock speed, and 4MB of cache. Reason being that whilst quad core is nice, theres very little software out there right now that takes advantage of dual core, so unless you moved into hardcore compiling and stuff since I last saw ya mate, I dont think its worth it. Also, as its clocked at 3GHz out the box, the chances are its fairly decent silicon, so you've got no risk with the 3Ghz, and it will probably clock higher fairly easily. Quad Core wise though, if you are interested, I think its the 2.4GHz C2Q they're dropping to $266, but as I say, I think for now dual core is more prudent than 4, due to the slow takeup of dual core.
A C2D, DS3, and the 8800GTS should do you happy

Im guessing you finally got a little fed up with the 7600GT
OS Wise, unless you fancy playing around with it (which I know you sometimes do), XP is currently still the better gaming OS; its currently still got better performing drivers, and has higher compatability, whereas Vista has some odd issues with varying games.