Let's pretend...

... That you're getting a Q9650. (Or have one, I suppose)

What motherboard, in your (knowledgeable) opinion would you choose?

I see a lot of people going with Rampages. Heard some talk about Biostar TPowers.

DDR2 is a better solution than DDR3 at the moment, in my opinion. Of course, the more the board can handle the better.

2x x16 PCI-E channels in Crossfire being a must.

As to whether its P45 (though I must say, I don't think I've ever seen any 2x x16 PCI-E xfire boards) or X48 is up in the air. I have heard that the P45s handle higher front bus speeds on quads, though.

Anyways, what's your opinion?

X48T-DQ6 - My current board. Love it. X48-DQ6 would be the DDR2 version.
P5E/P5E Deluxe - for a DDR2 solution and an x38 chipset. Can be flashed to Rampage BIOS.

I would go for X48 and if you want to overclock with 500MHz+ FSB then forget about P5E or most other X38 boards. P45 are better for dual cores. I haven't been using quad on my biostar so can't say much. For sure it's cheaper option. P45 boards have 1x16 or 2x8 pcie so bad for CF.
So simply look for some good X48 board; X48T-DQ6 seems ok but check some other boards/ read some reviews.