The Corsair CMPSU-400CX is a very solid unit with 30A on its +12V rail, and one PCIE power connector. It will have no trouble with a HD4850, which is right around $100 now. Other candidates within the budget would be a HD4770 or HD5670, but they do not perform as well.
Edit: GTS250 and HD4850 are on the same tier.
In the "next few months," card availability is likely to change. If you want a HD4850, better get it now.