I believe today that no game is even capable of using PCI-E's bandwidth to the max. SLI 8x is fairly good, but SLI 16x is better. However, the performance difference between is almost nonexistent. SLI 16x will probably be utilized more effectively in the future. If you are short on cash, 8x is fine. If you want to have slightly faster graphics or have cash to burn, then get a 16x.
Here are some charts I stumbled upon (It has both SLI and Crossfire results in 8x and 16x)
http://www.tbreak.com/reviews/article.p ... genumber=3