Any reason you are restricting it to nVidia cards?
GTX285 isnt worth it, its only slightly faster than the GTX275 for quite a bit more, i would expect a price drop on any of the nVidia cards for a little while, but the 5850 might drop down into that price range when Fermi comes out but Fermi certainly wont have an offering in that price range for several months.
So grab another 9800GTX card? You might even be able to SLI it with a GTS250, its the same chip. GTX260/275 is your best bet, or a 4890. Not sure the 5770 is enough of a boost to make it worth while. I'm actually not sure any of these are. What resolution are you playing at?
To be honest, I have to agree that buying a GTS 250 to SLI with your 9800 GTX+ would be your best path right now. The performance increase with a GTX 200 series card would be noticeable but it's not really gonna wow ya. As far as Fermi, those cards wont be out until March & they are gonna be expensive. In order to see an improvement over what you already have, you're not going to be able to get one of the cheaper cards that come out. At the very least, you're going to have to buy an upper mid range card to see any real improvement & since they are going to be new, they will cost quite a bit.