I hate when people say a dual core will not help gaming at all. All new games are/will be optimized for dual core. You also forget, what about all those windows processes that eat up cycles? Programs running in the background? With a dual core and a game that does not take advantage of dual core, all these processes/applications will run on one core, while you have the whole other core devoted 100% to the game. Ever have a virus scan kick in during a game? With a dual core, youll never even notice it.
I went from a 4000+ to a 4800+X2 and ill tell you, i noticed a BIG difference. Everything seemed much snappier and more responsive to me. I had email, internet, aim, skype, bit torrent, media player, all open and i coudlnt tell the difference. On my 4000+, i got real bogged down.
I say, if you have the money, go for it. You wont regret it, espicialy since thats the weakpoint in your system