750W should be good enough to start off with. I'm running 3 cards off a 750W right now. The card you'll want right now is this one:
http://www.newegg.com/Product/Product.aspx?Item=N82E16814102946&cm_re=5830-_-14-102-946-_-ProductYou can run 3 of them safely with that PSU (maybe 4, I dunno.. I never tried) which will give you just under 1 GH/s. Short of that, you should be looking for an HD 5870 or 5850 on the used market. I wouldn't pay more than $200 for the 70 and maybe $150 for the 50. If you want a decent mining AND gaming platform, skip the 5830s and try for the 5870s if you can get your hands on them. Else, get the 6950s which can be unlocked to give you 400+ MH/s and legit gaming performance too (BF3 is right around the corner!)