L1qu1d's right, your money would be much better spent on just adding another 8800GT.
An 8800GT is essentially the same card as an 9800GTX except with 112 shaders instead of 128, and lower clocks. It's probably only 15~20% slower and simply overclocking it up to 9800GTX+ speeds can reduce that deficit to around %10.
Ergo, spending only $110 will only be about 15% slower than spending $350 on two 9800GTX+s. 300% cheaper, 15% slower is hard to argue with.
If you really want the extra horsepower, spending $450 dollars on a set of GTX 260s would be a better value than $350 on 9800GTXs.