Frankly, I've used every leftover board too cheesy to run Windows on CC4/5, and rarely had a issue. But as part of making my home system (7 clients) a bit more reliable and lower maintenance, I've standardized on the Supermicro X7SLM-L for a MB. It's micro ATX so is very small, has simple VGA (GMA950) and audio, two GB ethernet interfaces (RTL8111C), USB 2, one IDE and four SATA ports. No floppy I/F. Standard Intel 945GC, ICH7R chipset. But it's server-intended, so it has a nice BIOS and plenty of fan controls. It iworked with CC5.0 fine out of the box, no special drivers needed for the basic installation.

I have two of these (a Gateway Mode/PDC with SATA RAID 5, and a single disk RSYNC mirror) in Supermicro 2U rack cases, which are rugged, keep them very cool, and can be found fairly cheap on eBay.

This motherboard is available quite cheaply. It does not run current high-end processors, but tops out at the Intel E4xxx family; still easily available on eBay. Your can put a new, cheap ($39) E1xxx dual core Celeron in it and it has plenty of power for a small network!

Mine has never so much as hiccuped once I turned off the Watchdog monitor BIOS feature. That's probably fine also, but I haven't bothered to set it up properly.