Double the bandwidth, little to no actual performance increase, in benches I've seen 2 7800GTX 512's in SLI x16 only saw UP TO 15% increase over regular SLI but I'd say its worth it FOR ME since the P5N32-SLI Deluxe happens to be my favorite Intel mobo out at this time, and it just so happens to be SLI x16 lol.
LOL in 3D Mark 05 0% to 1% improvement, WOW!
CoD2, SLI x16 is 5% BEHIND regular SLI, ***?
CoD2, AA and AF turned up, SLI x16 pulls ahead....a little.
etc. I think I've made my point, SLI x16 shows a LITTLE improvement only with higher resoultions and AA and AF turned way up, other than that there's really no point to it BUT I think if you're getting SLI, you may as well go all the way and be prepared for the future since newer GPUs will only use more and more bandwidth.