A 7800 GTX is very similar to a 7950 GT. Architecture is identical, the 7800 GTX just has lower clockspeeds. I'd estimate maybe 90% as fast.
(if it's a 7800 GTX 512mb it's actually faster than a 7950 GT)
Therefore, two 7800 GTXs in SLI are similar to two 7950 GTs in SLI.