I owned and sold a lot of those chips a while back.
Sounds like a tight budget and you did not say exactly what you are running so here goes.
If you have an Athlon 64, or Sempron... any single core chip then you will see a HUGE difference going to dual core.
5200 to 6000+ or whatever core will all be pretty good. some are better then others but not by much.
If you have anything better then an Athlon X2 4400... it is not worth the upgrade.
If you have 1 gig of ram, add another.
If your videocard is less a geforce 7300gt, 8400gs, radeon 1300 or similar,
Get a Radeon 4650 or 4670 and this will still manage to play games such as Company of Heroes, Starcraft2 even Crysis will be playable and look alright.
(eddited)
Would be best for you to give as much details as you can as no one knows what you really need...
For example, an athlon X2 5200 with 2gigs ram + a radeon 4650 can run a lot of games fairly well.
A friend of mine is still on my old old 939pin Athlon 3800X2 with 2gigs DDR400 + Geforce8500GT and he is more then satisfied... playing company of heroes and starcraft2
So if you balance out your system, you can milk it another year. (assuming you have XP as vista/7 need more ram)