I apologize for not seeing you stated the GTX 650 in your heading. I only read your post.
The GTX 650 looks like this compared to the HD 7750: http://www.hwcompare.com/13468/geforce-gtx-650-vs-radeon-hd-7750/
The 650 would be the faster of the cards, but would require a 6 pin PCIe power connector and a strong PSU. If yours only has a single 25W, +12V rail it is far from being a 'real' 450W PSU. It may hold if you don't tax the graphic card to heavily for too long. But if the PSU fails to provide the current or voltage when stressed, it could cause a shut down at best... or take out some other component when it fails catastrophically.