I’m going to buy a new screen in a few days. It’s native resolution’s going to be 1680x1050. Now I have geforce 8800GT so I’ll need a new one if I want to play newest games. I’m thinking about Geforce GTX 260 or Radeon 4870. Which one of them would be better in games (that’s what I mainly use computer for). Or maybe I should get a little extra money and buy Radeon 4890?
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.
The 4870 is definitely the stronger GPU at your resolution in most games. Don't let brand preference stand in the way of facts. Just look up the results for the games you will be playing at 1680x1050 and let the results speak for themselves.