You have:
1) an SLI motherboard
2) a single 8800GT
That should give you the answer right there. Pick up a second 8800GT for ~$100 and outperform the GTX 280 in >90% of games available today.
Otherwise (as jaredpace mentioned) a pair of GTX 260s or a 4870X2 are the only options that would be faster than 8800GT SLI (
benchmark). Those are much much more expensive though for not much faster performance. If you're seriously looking for best
value go for a second 8800GT in SLI and then wait for the next generation of cards.