thanks guys. Yeah a 5870 would be great if I came across one. I have limited knowledge of mini-itx, are there mini itx boards that support crossfire?
You don't need crossfire for dual card Mining.
You may use this mainboard: Gigabyte GA-880GMA-UD2H, 880G
or this: Jetway MA3-79GDG Combo, 790GX
or this: MSI 890GXM-G65, 890GX
That are µatx boards for AM3 Socket (latest AMD CPU's) with dual PCI express, If you like Intel CPU better please send me a PM or reply in this Thread.
Additional Info: Bandwith of your PCI Express slot isn't a criterion for mining Bitcoins, anything above 1x PCIexpress should be fine, without any speed losses in mining. For games it would be another story, but i assume that you don't plan to play games with you mining rig. right?