If you're only going to playing games, surfing the net, and doing office crap AND you want something fast at a reasonable price... then the Athlon 64 4000+ is your solution.
Now if you are into audio/video encoding, gaming, and other apps that take obvious advantage of dual cores, then an Athlon x2 is your solution.
Just my opinion. I personally have an Athlon X2 4200+ cause I do a lot more than just game.
Also keep in mind that future games will almost certainly take major advantage of dual core processors. Right now most games only use 1 processor.
-mpjesse