That's a heck of a nice system. I run Doom 3 and HL2 in high settings on a setup that's not nearly that good (see sig). I can't imagine you'll have any problems.
My thoughts already echoed by others is power supply and cooling. Here's a picture of a 6800 GT SLI setup
samelessly referenced from hexus.net. Note the cable that connects the two cards. That's part of the magic that makes them work together. (Read the first of two articles at Hexus
With each of those cards pulling in excess of 100 Watts each, power should be a concern. Also, you've got top-shelf parts elsewhere, so don't skimp on the power supply. A poor or over-taxed power supply can show up looking like a lot of other problems like bad memory or graphics. Go with a good brand like Antec TruePower, PC Power & Cooling, others that people are suggesting here.
Also, those video cards generate a lot of heat, so plan on cooling them accordingly. A side fan blowing directly on them comes to mind.