Always fun but I've got some Q's I'd love to hear if anyone has had any experience with:
1) can I pull Video (including HD) fast enough over a 100MB lan if I keep stuff on a "server" or do I build my 1TB Raid 5 Array on the acutal HTPC connected to my AV gear?
You can, but Video will be slow-to-start and glitch prone. I did use video over a 100Mbit LAN at one time, and it worked okay I suppose (but I certainly wouldn't call it "good" or "great"). I now have a high-quality Gigabit LAN switch and all is good. I'd look to moving to a Gigabit network if you're going to put the RAID on a remote machine really. You won't regret it.
2) The new Intel MOBO has RAID 5 on board - should I use this OR WinXP Raid 5 Hack OR Vista Ultimate (comes with RAID 5)?
Software RAID scares me. I'd look for a discreet RAID card with an onboard hardware RAID engine personally. Of course, those are a LOT more expensive than using software RAID setups...
I imagine Intel probably did it fairly well (though it's still gonna be software based -- done in the driver). nVidia's onboard RAIDs have been dismal and trouble-prone. Haven't really heard any positive or negative stuff about the XP Hack method or Vista, but generally those types of software setups are much less fault and migration tolerant than hardware systems.