I have a SLI setup, and I'm running MCE2005.
Pros:
I can run dvi to an hdtv, and still have 3 left over for monitors when SLI is off (I'm a developer, I do the multi-monitor thing)
I can take advantage of having multiple cards when I play games.
I don't have to use an old pci video card for the aux monitors
I want my PC to do everything. SLI and MCE is taking that to the N'th degreee.
Cons:
2 6800GT's are very, very loud. You don't want them in the living room. I have mine in the office, and a short dvi run to the living room. I can do this because they're adjacent.
SLI is is way overkill for a machine you'll mostly watch tv on. If you want the performance of two 6600's, by a 6800gt. If you want the performance of two 6800's, well, stand tall, I'm right there with you.
MOST 6800/6600 cards do NOT come with component out cables. Only a few cards do. That means you'll have to tie up a DVI/HDMI on your HDTV. ATI cards frequently allow for the component 'dongle'.
HTH