I ran Win XP64 until 1 month ago and it was fine with 4gb of ram.
Last month I bought a Q6600 + 8800GT. As many have pointed out a dual core is most likely better if you just run 1 instance of the game. But since I usually run 2 instances of the game at the same time and eq2 tend to take up 100% of the core resources I decided that I wanted a quad core to be able to run 2 eq2 instances, parses and teamspeak without any performance issues. Also my Q6600 overclocks fine to 3,3GHz on air cooling so performance is good. But if the newer 45nm quads are availible I would go for those. Also I decided to install Vista 64 ultimate and its running very good so far. No crashes yet, besides on Ekron in VP before the memory leak was fixed. But Vista is finally starting to feel mature enough to run without problems. The drivers seems to be fine in most cases with newer hardware.
So it all depends on how you will use your PC. Lots of 2boxing on same computer = Quad core. Just want to run your computer with your shammy and not much else go for dual core.
Just decide, buy and hurry back to us you damn slacker
Your brother in heals
Eyan