In some benchmarks, SLI'd 6600 GT's can outperform a single 6800 GT by a margain. However, instead I would get a single 6800 GT (which will be dropping in price radically very soon with the release of the new gen of GPU's), and put a second in later at a bargain price. OR: you could get a 7800 GTX, or wait until later this year for Radeon X900 to be unleashed. Depends what you want to play on your comp...
First, SLI is no gimmick. Expensive maybe for two top-of-the-range graphics cards, but well worth it for the immediate results.
Second, it is reletavely simple to install provided you have:-
Two identicall SLI ready cards
An SLI ready mainboard
A decent PSU
Some basic knowledge of Forceware (hardly rocket science