No. I will regret it 'cos if I spend the money on Win7, I'll have to wait even longer to buy the things I really need and want, like a washing machine, graphics card, monitor (I've got quite a long list so I'll stop there). I think the only piece of hardware I've got that has less than stellar drivers is the Nova-T 500 and loads of other people have been and still are using it and MP on XP quite happily, so if there is a problem I'm sure it can be fixed. I don't see any reason why Hauppauge would have put any more effort into making the Win7 driver any better than the XP one and they certainly had more time to iron out the bugs in the latter than they've had for the Win7 one. Anyway, apart from the Remote not working after resuming from hibernate, since I got the XP USB hotfix it's been working OK for me in XP for some time, both on this box and my previous one.
I have Win7 on my dev machine and XP on the HTPC and AIUI Win7 actually forces better drivers because of they way they now have to work where as with XP all bets were off and lots of sloppy code is around (goes for software too).
It does sound like the problems are more fundamental than with MP though. If IE is not loading 1/3 of the time this is either going to mean something is on its last legs (HDD or PSU?) or something is conflicting.
I am assuming that your HTPC is in daily use so you can't experiment with it without upsetting the misses? I would seriously consider removing all the hardware and do a clean install of XP. If you can replicate the issue with IE on a cold boot 1 out of 3 times then it would not be too difficult to establish whether this is a problem on a clean system.
Then I would probably install the hardware one item at a time and see if the problem arises then. Assuming everything is still stable then I would then try MP but without hibernate to see whether that is an issue.
You can always use something like Acronis to backup your current setup and it only takes a few minutes to restore an image.
Without going through all this there is little we can do with MP as the problems seem related to your setup and possibly your combination of software / hardware / OS / config that no-one else has got.