a gtx 650 is one of the worst gtx cards EVER, it will not play games at max
650 ti is better.
but a 7850 would be really good.
and i think u should get a new pc bcoz the 1.0 pci-e is just too old and i dont know how well it will hold up the current gen cards
I think all video cards are backwards compatible. They should run in a pcie 1.0 mobo without problem. Do not get a gtx 650 or HD7750. Try to go at least for a HD7770 or gtx 650ti. However, these 2 cards require an additional 6 pin power connector from your power supply. You can use a adapter if your psu doesn't have the connector, just make sure that your psu can handle the cards. If your psu can not handle them, then I think that HD7750 is better than GTX650. Post your PSU specs.