SFF = small form factor = small pc's there's plenty of small media centre pc boxes about & they limit you in terms of expansion cards, cooling, usb ports etc.
Mine is an Antec Fusion box with an Asus mobo, 2 x 500gb hdd, 2 gig ram, Creative 5.1 soundcard, xfx 8500gt gpu & a wintv 500pvr dual channel tv card (dual channel = watch one prog whilst recording another) Vista premium 32bit.
All in about £600 - took 2-3 hours to build and about 4-5 hours to set up. The thing with building is you can get it cheaper, so a more powerful system for your money, but you do need to know your way around as in particular with media centre, it will need some setting up post build in terms of decoder software for tv, codecs for video files etc.
None of this is particularly taxing & all the info is out there but still potentially difficult. I've built 3 media centres now & feel I know my way around but if you buy one off the shelf, then you get a warranty, support etc. So that can be the way to go if you're pushed for time or talent
I haven't used the Linux mce but I've watched the video & it certainly looks good, but then you will need a compatible system - I understand you can download the o/s & mce package in one go.
There is also Media Portal, which is an open source media centre & is very, very good. I've had it & loved it, however it is open source freeware & in my experience, works flawlessly provided you're willing to tinker fairly extensively with it. Having said that, I haven't been near it for a couple of years & my guess would be it's come on a lot since then, since that was what it would do on an almost weekly basis.
In a nutshell, if you get XP media centre, then you will need a tv decoder to watch tv - it doesn't come with one. I have used all the tv decoders & find Nvidia's pure decoder to be the best - although this is only my opinion based on my builds.
Vista come with a tv decoder so you can plug in your tv card & off you go. However, I would still use a third party decoder.
In either version, you will need codecs to play avi mpeg & wmv files. Use ONLY ffdshow tryouts, which is a free download. Do not confuse FFDshow with FFDshow tryouts - they are different things & whatever you do, do not use codec packs, they're more trouble than they're worth.
If you've swallowed it this far, then you are a step away from Movies, TV, music, pictures, video & on demand services all in one box on one remote. It pauses & rewinds tv, will work with sky & virgin media, comes with a tv guide - you can record entire series with the touch of a button, in short nothing comes close.