The radeon 4890 is the best $170 card on the market.
The Radeon HD5770 is the newest $170 card on the market (DX11, etc).
The Radeon 4870 is the cheapest $120 card on the card (matches the 5770 in perf).
As for your powersupply, it should be fine so long as the efficiency is above 70%. Check the amperage on the 12V.