Hi amigadave
That's a very nice system you got yourself there.
The choice of which host OS depends on which you use for your entertainment centre. If you run Linux as your host OS, with XP and Vista as the guests, you will likely lose the ability to record TV shows, and also your 3D graphics performance will plummet. This isn't anything against VMWare, its simply a matter of fact. I use VMWare a lot for work and at home, and think it's amazing, but there are limitations to what it can do. This would be the same for using VMWare ESXi as it doesnt support 3D or USB.
I agree that XP x64 is the way to go as the host OS. This way, you can have your 3D games and apps still running well in SLI, and you can also keep the ability to record and watch your TV on the computer. Vista, Ubuntu, and OSX can then all be ran inside VM's. If you like playing with OS's, AROS, Solaris, Novell, and many others all run well inside VMWare too.
I would suggest you download the trial version of VMWare Workstation. This will let you use the full product for 30 days so you can set up your virtual machines, and then after the 30 days, the free (built in) VMWare Player will let you still use those virtual machines without buying the full product.
I would also like to echo that I think the idea of going for a Raid0 stripe for your disks is bad. If one of the disks fails, you can say goodbye to everything on the computer quicker than I can say "argghhhh!!!". If you go with a Raid1, you will have roughly the same performance, but with the advantage that if one of the disks breaks, the other disk will take over and you wont lose anything.
Hope that all helps
Mike