My recommendations, Intel or AMD, doesnt matter, just make sure the motherboards chipset is known to be stable and reliable. Use quality components every place thats critical, motherboard & RAM especially. I'd recommend Crucial memory for their known quality, preferably ECC if you can afford it. A 3Com network card. For video card it doesnt matter since it'll hardly ever be used, just a simple one. Also a good quality power supply is a must as well, Antec, Enermax, etc.
I've got two servers at a co-lo, ones a PIII 650 on a Asus P2B-F board, with 512MB Crucial ECC RAM, 2x 36GB SCSI disks, it's been there for 2 years now with no problems at all. The others an XP 1600+ on an Asus A7V333 board with 768MB Crucial RAM and 2x73GB SCSI disks thats been in service for about a year now also with no problems.