As of now, I have this PC:
Athlon II X3 450 @3,3Ghz
Asus M4N68T-M LE
Kingston 4GB DDR3 1333MHZ
OCZ SXS2 400W
I was considering to buy a new GPU, however the HD6870 and GTX560Ti are very expensive right now in Brazil (and people say that upgrading to a 7750, GTX550Ti or even a 7770 won't be a good investment).
So, now I'm thinking about changing something else: CPU, Motherboard or both.
First of all, I was very interested in this Core i3 2120. However, to go for it, I'd have to buy a new mobo altogether. I could do that, but only if it's a good upgrade in terms of gaming.
So, right now I'm considering the other possibilities, for instance: getting a better AM3 CPU or trying to buy a Mobo with core unlock feat, which might give a little more performance for the best price.
I can't really determine a budget, since I'm talking about goods in Brazil, but performance-wise I guess we could achieve some optimized setting.
I just changed from a X3 450 to a I5 3570 with the same HD6850 and frame rate went up in excess of 50% have a feeling you might experience something similar with the I3. My daughters rig has X4 955BE and she got a drastic increase in going from 3.2GHz to 3.6GHz more than the 12-15% clock speed change.