I have grown quite fond of the Intel D945GCLF2 Atom 330 Mini-ITX motherboard. It can be had for under $100 USD, sometimes even under $80 USD. It's tiny, uses very little power, and while not a speed demon, it does pack a punch! So far I have had great luck with it under XP and as well as Ubuntu. I recently decided to give it a try with OS X. I was able to put together a full system for just a hair over $200 from NewEgg. That is case, hard drive, 2GB of memory, and the motherboard (which includes usb, audio, video and ethernet).

To get OS X running on this system, you can't just use a retail disk. If you want to give it a try yourself, Google for "d945gclf2 osx" and you will find several handy guides to get you going. The only real problem I have had was trying to set up Time Machine to do backups. I connected an external hard drive, partitioned and formatted, and tried to set Time Machine to use one of the partitions. This generates an impressive sounding error... "Time Machine could not be configured. The built-in network interface could not be found." I found this to be a bit baffling, as network access was surely working. Google to the rescue! I found a few tips to fix this.

The first was to remove
/Library/Preferences/SystemConfiguration/NetworkInterfaces.plist
reboot and reconfigure the network interfaces. No luck there.

The next recommended:

• Replace IONetworkingFamily.kext with a modified one
• rm -r /System/Library/Extensions/Caches
• rm -r /System/Library/Caches
• rm /System/Library/Extensions.mkext

This didn't work for me either. Plus, I didn't want to modify system files more than I absolutely had to.

Finally, I came across this trick:

• Download and run EFIStudio
• Change 'device' to ethernet
• Click Add Device
• Click write to com.apple.boot.plist
• Reboot

This worked like a charm. I was able to select the partition for Time Machine, and it has been running without error ever since.