Building HTPC, but what OS?

Although this may be better suited in the software section, I feel if I put it in a respective OS area, I will get biased results. I figure a good demographic will look here and be able to help.

So, I'm planning on upgrading my gaming box to a Core series processor and using my Q8200 and mobo to go in an HTPC machine. I know how to build it so they will come, but the one thing that is stumping me is what OS I should use. I've figured here are my choices:

1) Windows 7 Pro, $150. I'm familiar with Windows, but the con is obviously the cost.

2) Windows Server 2008, $unknown cost. First, what version do I need? how many CALs? I have 1 desktop, 2 laptops, 2 Blu-Rays, a Wii and 3 DVRs. Do I need like 2008 Standard with 10 CALs for each component that connects, or Home Server 2008? I'm really Server ignorant, as I've never used one before. Maybe some tips would be helpful.

3)Linux. Free. The biggest con here is I don't know Linux (I dabbled with FreeBSD many years ago, so I'm sure I can get it working, I'm just worried about maintaining/fixing if something goes wrong.) I'm pretty sure it will be Ubuntu with Myth-TV. I'm currently using Ubuntu dual booted on my lappy with Win 7 Pro so I can get used to the system in case I go that way, not to mention it's fun not using something MS related!!

I only need to worry about HTPC serving, no mail servers, web servers, VM servers, etc etc. Only other thing I might run is RDP. Can anyone give me some solid pros and cons to my dilemma?