There's no need for something outrageously expensive like a GTX 590 to play a game like Skyrim. The highest end cards are always simply terrible values for the money so unless you are wealthy enough that money doesn't really matter to you at all there's no reason to even consider that card.
I agree with ilysaml. Get something like a GTX 560 Ti for now. It's approximately 3x faster than your 9800GT. Closer to 4x if you get a card with a high factory OC or do it yourself. When the new cards come out figure out if any of them make more sense than adding a second GTX 560 Ti for SLI.