SLI 7800GT or 1x 7900GT?

Ok, so it appears that the 7900GT will cost a little bit more than a 7800GT
costs right now (vendors depending, of course, and this in Australia where
everything is a little more expensive video-card wise).

If the 7900GT slightly exceeds the base clocks of a 7800GTX (450/1320 vs.
450/1250), I'm assuming 2x 7800GT (in SLI) will outperform a single 7900GT.

Um...is this right?

Essentially - should I go ahead and buy a second 7800GT (leadtex extreme
450/1050)?

Or wait and buy a single 7900GT?

I think this might turn out to be a very, very stupid question...but I need
reassurance!