CPU upgrade with K9A Platinum Mobo (Help)

Hello all,

I currently have a K9A platinum mobo from MSI and I've had it for a while now. It's currently too costly to switch to a new mobo/CPU and buy the new DDR3 RAM so I'm looking into possibly upgrading my CPU.

Doing some googling, I found out that my motherboard is a Socket AM2 type, and that the AMD Phenom is supported with this motherboard. My question to you guys is, what is the highest-end processor available for this motherboard, that I could just install without buying other new parts?

I have a GTX280 card, with 4 gigs of DDR2 RAM and I think my AMD Athlon XP 5600+ is slowing me down. I also recently tried out Windows 7 Ultimate but I really don't like it. I think it's slowing down my games and I'm not really that interested in all the new "features" of it. I'm thinking 64-bit XP right now, as 7 is simply too slow and cumbersome. I really don't know what I'm going to do once XP becomes truly obselete but Vista was a disaster and 7 is only slightly better. Anyhow I'm trying to make this powerful for games, so keep that in mind, and let me know what you guys would recommend in terms of a new CPU.

Thanks much!