I would say so. (2) GTS 250 would outperform a GTS285 anyday of the week.
Are you looking to buy (2) or add 1 to a current card? If you are adding, its an awesome upgrade. If you are going to buy 2 off the bat, I'd go for a single card solution, a 5850 most likely. But that's just my thoughts.
Depends on the game and the settings you use. Memory might be an issue as you are using 512MB cards rather than 1GB cards. To get an idea, you can check out the charts here to see how they compare to other setups. They use two GTS 250 1GB cards, but you can compare to the 9800GTX 512MB cards to see where memory may be a limiting factor and if performance is what you feel comfortable with.