If your not a avid hardcore gamer, then if you could find a cheap X2 CPU then a 5000+ (or as far as your motherboard allows) and a GTS240 or GTX460.
This will tide you over for a little while whilst if you want, save up for a whole new system upgrade. (Mobo/CPU and/or RAM if you decide DDR3)
Ive just done this for my grand dad, he had the X2 4200+, upgraded to a X2 5600+ and a GTX465 and is perfectly happy with the results.
Well, I've checked the list of compatible CPUs for my mobo and found Athlon 64 X2 Dual-Core 4800+ is supported. However the price for s939 is insane (over $200 used), thus AMD Athlon 64 X2 Socket 939 4200+ Dual Core comes in play. It's much cheaper. BTW, I've checked with forums and testing graphs show that there's not much of a performance difference between my current CPU and these to dualcores (not much but dualcore). Or did I miss something?