Not sure about the prices, but the 660ti performs about the same as the 580, while the 660 is in par with the 570. The 650 is much weaker, about the same as the 550ti, even weaker than my old ati 5830, what isnt a powerful card at all.
I would check the HD-7850. Same performance as the 660 or the older 6950, only eats 140 watts. Also, in Europe, the ati cards are better in price-performance wise than the nvidia products.
Also, that CPU (dual core pentium, right?) looks pretty much weaker than the new i3s. Not sure if it can drive a 660/7850 level card withouth being a serious bottleneck.