I've had good results with the Asus A7N266-VM/AA at work, where I tend a fleet that includes about 30 of them. I even owned a couple of them myself, one for work and one for home. They're a bit fussy with Windows98, but with Win2000 or WinXP you can count on a pretty smooth ride.
At about $56 shipped from Newegg, they're well-priced and come with high-performance audio that is usually compared to the original Creative Labs Audigy card. They are microATX and do happen to include onboard LAN and onboard video, but also have an AGP slot. One of the things I like is that they are very stable and reliable, judging by the 30 that we have and the two that I owned myself. Granted, it's not cutting-edge or even close to it, but it's cheap, proven, and stable. If you can swing a little more money, get a Shuttle MN31N and make sure to use two memory modules so it can run dual-channel mode, and then you'll have a GeForce4 MX onboard for free, plus USB 2.0 capabilities, overclocking capabilities and more of an upgrade path.
If you did pick the A7N266-VM, you would want unbuffered non-parity (ie, "normal") PC2100 memory (Crucial works well). You could use a microATX case or a full-sized ATX case, and I would vote for a full-sized case with a respected-brand power supply in the 300W+ area so you don't regret it later when you get some whopper top-end video card that pulls 60 watts by itself. Inwin has one with a memory-card reader that I like the looks of:
link No, it ain't cheap, but there are two meanings of "cheap," aren't there.

Ah so... buy the right thing the first time, is what I think.