7900 GS 256MB SLI or 8600 GT 512MB SLI or 8800 GTS 320MB

The 7900GS and 8600GT are close enough in performance that I would base my decision on price. That said you're better off going with 8800GTS since there are many cons to using two cards, and few benefits. The two key factors are the resolution of your monitor and what games you play. The SLI/Crossfire FAQ covers the cons pretty well...

http://www.tomshardware.co.uk/foru [...] sfire-faqs

You can also compare the scores for the various cards in the VGA charts (in the dual vga charts the 7900GS's should perform a hair better then the 7800GT's).

VGA Charts
http://www23.tomshardware.com/graphics_2007.html

Dual VGA Charts
http://www23.tomshardware.com/graphics_sli2007.html

Thanks. So I guess a mid range card, even when configured in SLI don't stand a chance to single high end cards.

Probably not lol. The 8800 is that powerful! I too would go with 1 single 8800GTS as it uses less power then 2 7800's in SLI or the lame 8600 cards, less noise, less heat in your case, and close to the same FPS.
Though a person would have to be very desperate to buy a 8800GTS 320mb over the far superior 8800GT which costs about the same!! Though they are hard to find now, they will be more plentiful as time goes on.

7800, 8600, 8800GTS 320mb-----If they really are your only options right now, then say fudge it and wait until you have better options, unless you can find em super cheap.