Overlord, Overlord: Raising Hell and Overlord II are all now available on Steam for Linux. Overlord II is also a brand new port today!
Note: You may have issues if you previously purchased them from bundles and other random stores. VP has reached out to Codemasters to add Linux to them. If you're only buying it now, you will be fine.
You can see my previous thoughts on Overlord and Overlord: Raising Hell in my previous article here.
I think Overlord + Overlord: Raising Hell are well worth the money and Virtual Programming did a cracking good job on them.
As for Overlord II, it's a brand new port! I had even shorter notice than last time being told only on the day it's released. So you will have to wait until later tonight for any thoughts on it. In future I'm sure I will get more time to cover VP's ports properly.