For a mild(3.3) overclock, only a decent cooler is required.
No X48 mobo's support i7; all X58 boards do.
To get 24gb of ram support, look for 6 ram slots. When 4gb ram sticks show up, then 24gb will be supported. Some boards, like ASUS may not now show 24gb support, but they will do so with a bios update when 4k ram sticks are available. Chelk the manufacturer web site and it's forums to verify this for any board that you are considering.
Nvidia wants a license fee to enable sli support, and some boards are cheaper vecause they do not pay it, and therefore only offer crossfire support.(sli is for nvidia cards, crossfire is for ati cards) Check carefully for this if sli is important to you.
Look carefully at the board layout to see if the requisite number of vga cards will fit. High end cards are double slot units, and they need an empty slot next to them.
I think I would pick the manufacturer first, basing the decision on how I felt about their quality, and support. Cards that have been out a while will have had the inevitable minor bios fixes already applied. My short list would be ASUS and Gigabyte. Look for features that you need. Hoe many SATA ports do you need? Some will include SAS support.
If your desire for 3 way sli/crossfire is based on gaming, then you should reconsider. There is very little gain from a third card, and even the possibility of a performance decline. A single GTX295 will be simpler and stronger than many multi card implementations.
Windows 7 seems to be getting good reviews. I have beel fooling with the release candidate, and my limited testing works well. You could install the free release candidate in june, and plan to do a re-install with the official release when it arrives around october. If a new clean install after a couple of months use, or the possibility of bugs is a problem, then vista home premium 64 will work fine.
If you are interested in using the vga cards for it's non vga processing power, then many might make sense.
If a case can fit a single vga card, which can be 10.5" or 11" long, then it can fit many of them.