^+1 for the 5670.
Even a 5670 will be bottlenecked by your old CPU though. Since you have an AM2 board you may also want to look into upgrading to a faster CPU. The BIOS probably wont' accept AM3 or AM2+ chips, but it should still accept most of the older Windsor and Brisbane chips. I'd check for BIOS updates and see about perhaps an Athlon 6000 or 5400. If you have the default 1GB of RAM though I suggest you upgrade the RAM first and then upgrade the CPU only if you have money left over to do so.