Will Radeon 4870/4890 bottleneck in GIGABYTE GA-G31M-ES2C (rev. 1.x) ?

Cna you provide a pointer to the GB site for this board? My manual collections shows the ES2L sharing a manual with the ES2C, but, hunting at my usual selection site:
http://www.gigabyte-usa.com/Produc [...] +775#Intel G31
doesn't show it, and their search feature gave no result. The manual I have shows a PCIe2x16 slot, which is as fast as it gets (good up to 250MB/s)...

Yup - still shows a 'shared' manual; my understanding is that Intel's GMA3100 is absolutely ass-kicking, as integrated graphics chips go; but, if it's what you were planning, I don't believe it will do HybridCrossFireX with your 4870/4890; need an AMD graphics chip for that - they have a new 4200 that also seems to kick booty!

I guess I'm not understanding this; my question is: why do you think it will? It's not like there are two slots, with one of them at 4x or 8x - which causes endless questions about the matter - and the fact is, very few graphics boards will even 'fill' a 4 lane PCIe - take a look here:
http://en.wikipedia.org/wiki/PCI_Express
for an overview... An eight lane slot might start reaching capacity with a pair of dual GPU cards, i.e., a couple 4870x2s 'talking' but, you have to remember, if they're crossfired, some of the bus loading is passed off to the crossfire connector. There is nothing on the market (and likely won't be for a while) that will overrun a sixteen lane rev2 slot; and when they do get that far, rev3 will, once again, double the signalling rate!