5770 is around the same as a GTS450 and the GTS450 is a little better than a 9800GT+, therefore if I was in your situation I would definately for for SLI. You just won't get DX11, that's the only drawback.
Consider selling your 9800 and getting a 460 or something? Maybe even a 580.
Hey i updated my Signature. My power supply can handle it, it has another slot for a card. Motherboard can handle it. Just i want to play Battlefield 3 at the end of the year and i believe that is Direct x 11