You'll need to check if your power supply has Pci 6 pin connectors. These cards need at least 1 each. All modern cards require power.
I'm a recent convert to ATI's new range. Much better bang for buck. If you insist on 2 cards then 2 4850's would give blistering performance.
Personally, i use a 4870 1Gb which plays all games at max settings. Then in a year i would sell it to buy the next latest card.
For your budget you could get Nvidia's 260 or maybe 280 which would be a much better use of your money if you still want Nvidia.