Wow thanks all! Got lots to be thinking about there.
I'm still not 100% decided on the 8800GTX idea but I would quite like to. I know it's not typically what an HTPC is for, but with a big telly and a nice big couch, it would be great to play games on that! And I have loads of games already, and also have the card already. I can just do the easy option and skip all that and use a cheaper smaller card.
I would like to still do it if possible though. It will be on 24 hours sometimes, but 24/7 isn't really likely. For example, I may have it on for a whole weekend, but not then for the rest of the week.
What I'll probably do is look at the cases people have linked and see if there are any that would work well for me and my price range and stuff. If not, I'll have to decide between a tower case on it's side, or just forget about high end gaming on it, and get a nice slimline case and just use it for web and music and video. Which would still be nice =)
Thanks again.