for a good Crossfire overclocking mobo, i hear the abit AT8 32X is pretty nice. I had the AT8 16-lane version, and i think i had a buggy board but other than that it was really sweet. I've heard good things about it, and the RD580 is supposed to be able to hit 300 FSB without adjusting the HTT multiplier. I'd steer clear of RD480 based boards, i've heard they have some issues with crossfire and memory compatibility
As for other boards, DFI's Ultra-D and SLI-DR are both good choices. Also, Abits more recent boards seem to be pretty good. The AN8 32X has been getting great reviews, and i have one and have really liked it. The only problem is that with the 1.0 bios, the HTT bus doesnt like to boot any higher than 280MHz. With the beta 1.1 bios though, i've heard it hits like 325, and i even saw 1 review where they hit 345!!!! I've heard that ULi board Effect was talking about is good too, although i'm not sure how well nvidia's driver support will be now that they bought out the company.