microATX is fine. My suggestion is to resist the urge to succumb to cheapitus peripheralis if you're building office systems, they're going to have to last a while (right?). Quality-brand PSUs, quality-brand RAM. Otherwise get some HP d325's and wash your hands of it.
If I were doing an office rig today I would probably check out those Asus K8S-MX's with an A64 2800+ and one 512MB Crucial PC3200 module, a Seagate 80GB ATA/100 hard drive, a Lite-On CD-RW/DVD-ROM combo drive, and an Antec SLK3700AMB case with Antec SL350 power supply.
edit: actually, in this case I might use an SATA drive, now that I recall the board's only got one IDE channel.