I don't understand the comment about Intel and SMP. Linux long had the reputation of having crappy SMP support due to scheduling issues, but I've not heard of any intrinsic problems with Intel SMP itself.
Now, older PCs of all varieties tended to be flakey by industrial standards. But good BX/GX based boards have proved to be extremely reliable, both single and dual processors. The i840 boards are apparently very reliable, too - in their (expensive) RDRAM version.
The main issue in regard to motherboards is the manufacturer. ASUS is has an extremely good reputation (HP uses them), Supermicro boards are really solid but have a rather kinky BIOS (I own two of them), Intel boards are conservatively built and solid.
Patrick does make a good point about memory support. Make sure any board you look at supports four DIMM slots if you want maximum RAM support.
