you're already too outdated.
If your motherboard is socket AM2+ then you can drop in a new Athlon II or Phenom II and be OK (but probably have to get the lower power versions - I doubt your board can handle to top-end hot 125W versions).
If it's a socket AM2 board (like I suspect) then any processor upgrades will have negligible performance increase and thus is a waste of money tbh.
Download CPU-Z and see what it says about your motherboard - if it's AM2 then save your cash for a new system. You could get a AM2+ motherboard for a new Phenom II and reuse your RAM, but DDR2 is getting expensive as production winds down (not that you'd need more than 4GB anyway).
Seriously, if you're happy with what you have now, start saving for a...