If it is a new build then I'm with David, get the best GPU you can afford instead of buying 2 cheaper ones.
Though I'm assuming this is for future reference for if you ever want to run 2 cards?
Most motherboards actually default to x8/x8 in SLi/Xfire unless you are buying a high-end board.
As to the difference, here is a good thread on it: HARDOCP - Conclusion - GTX 480 SLI PCIe Bandwidth Perf. - x16/x16 vs. x8/x8
It seems that at 2560x1600, even with 4X AA, there was absolutely no difference between x16/x16 and x8/x8. This is good news if you game at x8/x8 on a single display configuration at 2560x1600 and below. You simply are not missing anything, and moving up to x16/x8 or x16/x16 will yield no performance improvements or gameplay differences, even on the fastest GTX 480 SLI.
However, if you game in a triple display setup at 5760x1200 or higher, then you might want to aim for more PCIe bandwidth. It seems that you will at least want your primary video card to be running at x16 mode. At x8/x8 we found "benchmarkable" differences in performance. We say "benchmarkable" because these differences were only noticed on a graph.
Please read through the thread thoroughly before responding.
Nobody wants to read a wall of text, formatting is your friend.
i5 3570k | 8GB G.Skill 1600MHz | R9 290 | SeaSonic 750X