I got a significant improvement on my 670's.

I'd really not suggest you go out and try to find a different 630. You'd be better off checking if your power supply can handle PCI-E power (usually listed as PCI-E 6-pin connectors), and making sure you have enough space in your case. Look for something like a 550, 660, or 670 (though a 670 can be overkill...)