What kind of motherboard and RAM do you have? If you're planning to upgrade to Phenom II or Phenom, only few AM2 boards can support it. But in gaming, it's better to use dual-core CPU with higher clock (3 GHz or more) than triple or quad-core CPU with lower clock (< 2.6 GHz).

Am currently running an AMD Athlon 64 X2 5000+ Black Edition, overclocked to 3.1 Gig, on my Abit AN-M2HD, with a relatively cheap ATI 512MB PCI-e vid card. Love it. The 5000 overclocks easily, and is stable as a rock at 3.1 GHz. Getting 60 fps in WoW. Anyway, these processors are inexpensive now - got this for under fifty bucks at Newegg. Wonderful setup. Why spend the money for quad core when there are perfectly fine dual core AMDs out there for a lot less?

An AMD X2 3800+ won't be powerful enough to be paired with GTX 260 and become a bottleneck in overall system. Even X2 6400+ isn't good enough, try upgrade to Phenom II. But pay attention to your motherboard, is it capable to run Phenom or not.

I've bought a new motherboard and a phenom II 710, so I'm marking this thread as solved. Thanks for your answers.