definitely using of x16/x16 sli with high end graphics gives you an 100% performance while x8/x8 is not behind from x16/x16 (x8/x8) gives you an 98% becuase of brandthwidth but i'm telling you x8/x8 is a lot better idea it doesn't occur any problem or any type of trouble you cannot feel the difference between x16/x16 o x8/x8 everyone are using x8 configuration......
what i7 cpu and motherboard are you getting?
sandy bridge i7 (2xxx) has only 16 pcie 2 lanes. it can't support dual x16 without and nf200-something chip. sb-e i7s (3xxx) have over 40 pcie 3 lanes - those can natively support dual x16.
anywho, you might not notice any performance decrease if you go with dual x8 sli. the current cards don't saturate x16 or x8 bandwidth. you'll notice performance drop with x8+x4 (not sure if that's even possible) or 16+x4 modes.
Are any of these SLI? Or is SLI x16/x16 only.. Is x8/x8 considered sli?? And finally, can I run any of these at say x16/x8?
yes, those are. hell, even x16+x4 is sli..technically.