It\'s as worth it to get 2 video cards as it is to buy an expensive CPU: you get what you pay for.
2x6600GT = roughly 6800GT performance, little higher/lower depending on the benchmark.
At the time of release, 2x6600GT cost about the same price as the 6800GT. ($400)
SLI has 2 major selling points:
A. Incremental SLI. Buy $200 6600GT, have fun. Buy second 6600GT, now $~100~, a year later. Have more fun.
B. Brute force. Nothing can touch 2 7800GTX\'s in SLI right now. Nothing. It\'s also about $1,000.