You can only add the exact card that you have right now. I'd suggest getting an eVGA 6800GT because they are highly overclockable and are able to fairly easily hit 6800Ultra speeds and sometimes beyond. So in the future you'd have to get a 2nd eVGA 6800GT for it to work.
Also like everyones said, I wouldn't go with ASUS. IMO like beedub said, their quality has simply gone downhill in the last few years.
ABit and DFI are among the two top most reputable overclocking boards with lots of options in the BIOS. You may not overclock now, but time and time again I've seen people say they didn't want to OC, buy cheap boards, and down the road decide they want to experiment with overclocking.
However, ABit doesn't have a SLI based board I'm aware of yet.
DFI has a couple
The lanparty nforce4 SLI
and lanparty UT Nforce4 SLI
That's the UT version.
Also, if you plan on overclocking RAM perhaps in the future, the best brands to get would be
OCZ Platinum Rev2
G.Skill LA series
Corsair will give you some room, but not as much as the others.
Read that article as it explains why some RAM overclocks better than others.
But yeah basically that machine you're building will lay complete waste to the dell. You should notice an amazing increase in speed and performance