80%? 96% ???
A PCIe lane is 5Gb/s. Period. To figure out GB/s, you, like almost all transmission schemes, lose two bits per frame, so you divide by ten. One lane = ½GB/s. Four lanes = 2GB/s. Eight lanes = 4GB/s. Sixteen lanes = 8GB/s. Period.
No board I am aware of 'splits out' the ICH lanes to anything but x1 slots - they are mainly used up for peripherals: LAN chips, extra SATA controllers,, and, as I mentioned, any x1 slots the board has.
The 58's two sixteens are usually on a 'bifurcation switch', which allows the board to 'negotiate' with its installed cards to do: 2 x 16, 1 x 16 + 2 x 8, or 4 x 8. If you put
anything into these slots, even a x1 card, it 'uses up' at least eight lanes - it's how the 'bifurcator' works! Very few MOBOs are physically set up to support four double-width vidcards. Most of those that do, get four sixteens through the use of nVidia NF200 PCIe switch/multiplexors. You cannot put two X58's on a board; there are boards (server/workstation class) that do get seventy-two 'real' full-width, 'raw' lanes (
Tyan's 7025, for example) through the use of a pair of 5520 IOH's (the server/Xeon world's 'equivalent' to the X58), but, as a rule, these
do not support SLI/xFire - the trick to the 'twinning' is that the 5520, unlike the X58, supports two QPI links. These will also
not accept an i7; they require Xeons which
also support two physical QPI...
One issue that you need to know, to decide what to spend, for 'how much board', is the vidcards themselves that you intend to use.
Very, very few vidcards can actually 'use up' a sixteen lane slot - it's a hell of a lot of bandwidth! In the ATI world, it's pretty much limited to the 5850 (barely...), 5870 (especially the 2Gig cards), and 5970. For nVidias, I'm not as clear, as I'd not fiddle with one, but I'm pretty sure the list starts at the GTX-285, and includes the GTX 470's and 480's... Another thing to be aware of, is that if you wish to use four double-width cards, without water-cooling them, you typically need a fussier case - at least eight slots, and, for many, like the GB UD9, a nine-slot...