The usual suspect would be ECS K7S5A. Runs up to XP2600+ CPUs, has onboard LAN and sound, and offers the upgrade path to DDR RAM. If it has to be REALLY inexpensive systems, you can use their M810L board with integrated-everything, there even is a M810CL model that has a (non-upgradeable) Duron CPU and fan right out of the box too.