PCI-E 2.0 doesn't matter a whole lot, but having two full speed 16x electrical lanes definately does if you're interested in Crossfire!
To the OP, one of the 16x lanes is often labelled in the manual (or sometimes right on the motherboard itself) as being the "A" or "1" lane which I'd recommend using. It probably doesn't make a difference though which one you use.
This might not a bit of another question but it could be considered in the same topic so:
Does anyone know a 775 board with 3 PCIe slots for crossfire?
I found the x38 Asus maximus extreme has one 2.0 16x and two 1.0 8x but I'd prefer to get an Nvidia style 16x PCIe on the 3th slot board for crossfire.
The idea is to buy two 4870's next month and ad an X2 version later this year.
I think right now you need to buy the AMD 790 (FX?) chipset to get three or four full speel 16x lanes for ATI cards...which means getting a Phenom, so yeah. But otherwise, I think you're only other choice is to get an Intel Skulltrail motherboard which to me is way more ridiculous than a Phenom chip lol
I was afraid of that, I'm not looking for an AMD board (Crysis eats CPU's) and skulltrail is not for my budget.
An X48 with an improved crosslinx technology (like the Nvidia 3way boards) of the Asus Maximus Extreme would be excactly what I need.