Don't worry about lane speed, you can check out this article
from Tom's, but basically this is what you want to know (taken from that article, (c) tomshardware.com):
While we did see a fairly large difference between x8 and x16 slots when a single card was used, adding a second card shifted our limit to CPU performance. That is to say, for most of today’s tests, a faster CPU would be far more important than dual x16 slots in achieving the ultimate SLI performance.
That answer presents its own set of questions, since our high-flying Core i7 CPU was already pushed to 4.00 GHz. Most builders simply can’t go much higher with a daily-use gaming machine.