None of the three will do SLI or Crossfire correctly. Only the first PCIE-x16 slot is actualy x16 on those boards...the second slots are all x8.
I believe the cheapest DFI that will do true Crossfire is the DFI LP LT X48-T2R...it has two full speed x16 slots plus one x4 slot for CrossFire + Physics at x16/x16/x4 bandwidth.
I'm not sure if Gigabyte makes a dual x16 LGA775 board, but they make some very nice I7 boards with dual x16.
Stay away from Asus...their quality has become terrible since they became an OEM.
Contrary to what Killerbug says, P45 boards are fine for crossfire. I can even run quad crossfire reasonably well with 8/8x, the effect is there, but it's nowhere near as bad as you might think. You are restricted to crossfire with P45, you can only use SLi with an nforce (and they're crap).
The EP45 boards are better than the P5Qs really, but neither are terrible. The Biostar TPower I45 is another worthy contender, as is the DFI T2RS.
The best boards for crossfire are X48 chipset boards, but Gigabyte have some PCI express Q&C issues with those boards, and Biostar don't make an X48. Your best options are an X48 from DFI or an Asus Rampage Formula.
