A8N5X - socket 939.
Supports u pto an FX60 (dual core 2.6Ghz).
What have you got in it now?
These cpu's were raplaced by the AM2 (socket 940) cpus and only support DDR1 Ram.
You will have to look around for a faster CPU for it depending on what you have.
If you have a dual core already I would overclock it ... and perhaps spend money on a better graphics card.
You will otherwise need to upgrade the mobo, cpu, and ram, to get into something a bit faster.
The current Intel 8 series cpus are a good idea for an upgrade - depending on what you want to do ... more detail is required if you want more advice.
Good luck.