Ah... This NVIDIA vs ATI thing is driving me nuts! I considered the 7750 first simply because my current GPU is ATI and it's been working perfectly for years. But then ppl told me I should go for an equivalent Nvidia instead and showed me hwcompare... So after a week of limbo I finally made up my mind but it seems my decision was based on a not so reliable source! As far as I can tell by the product pages the only difference is that the 650 has 1100 Mhz clock speed, and the 7750 only 880. Doesn't that make the 650 a better choice?
I'll do some more reading on these two before I buy.
Thanks for the PSU-related replies! Looks like either GPU will be fine with my current setup. Phew!