Obviously a bit late to the party, but here's a few thoughts...
RE Gaming: by all means try something like this for casual gaming, but it's first and foremost a HTPC rig.
RE vs PS3: Sure, you could get movie playback over a PS3, but picture this...
- You've got all your movies stored on a central hard drive
- You've got a server running, say, My Movies
- That server might be a Windows Home Server, also running DVB link, and have a pile of TV tuner cards in
- That server might also have all of your pictures and music on
Now. Ad some of these bad boys to the equation (the ASRock machines) at every TV in your house, and you've got...
- All of your movies available on all machines via a cool jukebox interface
- The same TV channels available throughout the house
- Potentially the same recorded TV available throughout the house
- All of your music available in each room. Same for pictures
I'm not saying don't get a PS3 - by all means do, but this HTPC route over networking opens up whole new avenues. Personally I'd get both, but stick to the PS3 for gaming.
I'm currently running some of the above configuration, but with HTPCs that I've built myself. Now, these ASRock boxes have started to come down in price a little (it's a year on, after all), and they still fit the purpose. They're now cheaper than building your own.
RE Windows 7 choice: See above. I'm sure you can do most of what I'm doing for free, but that's the route I've chosen.
Just a few thoughts, and thanks for the review - it's made my mind up.
Cheers, Simon