What's the difference between running CrossFire on x8/x8 and x16/x16?

The southbridge of mobo is what controls your pci slots. It will have a set number of pci lanes. pci x1 uses 1 lane. pci x8 uses 8 lanes, pci x16 uses 16 lanes. you get the idea. The more lanes that a pci device can use the faster it can comunicate with the southbridge. However just because a videocard is compatible with pci x16 doesn't mean it can keep up with the max speed of 16x. (example: driving on a highway with a car that has a lower max speed than the speed limit.)

If you are running a mobo w/ 20 pci lanes and you are cf'ing you video cards generally you will end up with 8x and 8x. It may result in a slightly slower communication with the southbridge for the individual cards but as they are crossfired your overall graphics speed will be much faster.

example: 1 videocard at 16x with 50% utilization of the pci lanes will be much faster than 2 videocards at 8x with 100% utilization.

k ..so it'll run ..that's what I wanna know ..
I'm thinking bout getting a gigabyte 870a ud3 ..unfortunately its got 2 pci x16 slots one at x16 and the other at x4...I might be installing a second Radeon 6950 later ..how's it gonna be ??

Not that great it will work but you're much better going for a mobo that runs at x8x8 the performance penalty compared to a x16x16 configuration isn't bad really.
This article doesn't pertain directly to your gear but will give you a rough idea what to expect.
http://www.tomshardware.com/reviews/pcie-geforce-gtx-48...
http://www.tomshardware.com/reviews/amd-crossfire-nvidi...

The x4 speed for the second card will slow performance at 6950 performance levels so you'd be better off looking for a MOBO that runs a x8\x8 instead of x16\x4 -- the x8 lanes will not be fully saturated in most cases but the x4 lanes will be which will slow both GPUs and affect performance.

so would two 6950s on x16/x4 be less powerful than a single 6950 or if not less than two 6950s on x8/x8??

could you recommend any sub 100$ mobos for an am3 CPU ..with two x8/x8 pci slots, DDR3 16 GB max 1600mhz, USB3, 6+ USB2, 4+ SATA, 1+ esata, some pci x1 slots ..and reasonable oc headroom ..gigabyte, Asus and msi prefered ..thanks in advance ..  

Don't know if you'll find one for under $100 with all of that but the Gigabyte GA-890GPA-UD3H @ $135 has most of that for just a bit over $100.

thanks ..Ohh and one other thing..would ya by any chance know how to use those USB header brackets to get extra USB 2 ports ..like here its got 4 ports on the rest panel but supports 8 ..

great seems perfect ..just that there arent so many USB 2 slots, just 4 in the back ..can I use hybrid CrossFire on a 6950 ?? with the integrated solution that is ?

great seems perfect ..just that there arent so many USB 2 slots, just 4 in the back ..can I use hybrid CrossFire on a 6950 ?? with the integrated solution that is ?
and I'm getting a ViewSonic HD moniter that I don't think has an HDMi ..the DVI D here is good nuff ??

As far as the USB rear ports, the GB has an additional 2 USB 3.0 ports that are backward compatible with 2.0. Hybrid X-fire with the integrated 4290 is limited to a HD3xxx series, which is worthless. 6950 needs another 69xx card. Preferably the same (6950). Good for future upgrade option, but not as good as a single higher performance card.
As for the monitor, HDMI and DVI both send the same digital signal.

wait ..you said that two 6950s on CrossFire at x8/x8 wouldn't be as good as a single higher end card ..
the 6950 is already a higher end card ..all the better cards beyond this are just slightly more powerful ..so how would 2 6950s be outperformed by say a single 5970 or GTX580??

There aren't too many higher end than that, I am refering to 5870, 6970 cards. Mainly, I wanted to address the hybrid x-fire issue. Two 6950's X-fired would be about the equivalent of one 6970.

what ??!! ..jeez ..am I new to the CrossFire/SLI scene or what   ..what happens if I flash both cards and make em 6970s ..would that exceed a 580??