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Well, after much hardship, I present to you "Vibe", my beautiful little custom-built 1U rackmounted live-pa PC! :D 
She's the little orange hottie at the bottom of the battle-ready live rack...
Vibe is a sexy little beast, with a set of particularly excellent hardware: - ASUS M2NPV-VM motherboard
- 2G Corsair "CM2X1024-6400C4" 800MHz DDR2 memory
- AMD Athlon64 X2 3800+ dual-core CPU
- 100G Seagate Momentus 7200.1 RPM 2.5" laptop hard drive
- 420w CAI power supply
And I mean LITTLE too - she's a grand total of 1 rack unit tall (duh), 19" wide (double duh), and 13" deep. The 13" was *extremely* hard to find - the average 1U rackmount "server" PC is about 24" deep, far too large for any flightcase. I "ruggedized" this machine as much as I could - originally I wanted to have the machine booting using a 2G compact flash drive, but for some reason I just couldn't get that to boot. Instead, I chose a 100G laptop hard drive, for a few reasons - for one, it's smaller and generates less heat than a larger drive. For two, laptop drives are built to withstand significantly more bumps and knocks than your average desktop drive. Three, they draw a little less power, and most importantly, four, they take up less room, which allowed me to be a bit more creative with my mounting options. I used longer screws and mounted the drive through a set of four rubber grommets, effectively "shockmounting" it. 
For every connection that looked like it could possibly shake loose, I broke out the hot glue gun and gooped on some glue. This stuff is great - it's non-conductive, holds tight, but can be peeled off relatively easily with the help of a razor, if that's what you need to do later on. 
For recording, I picked up a 10k RPM Seagate drive, and mounted it in an external SATA enclosure, with an eSATA connector. I had the eSATA bay in the single PCI bay, but decided that bay could be better put to use with an extra two USB2 ports and FireWire ports, so I dug out my Dremel tool and cut a hole in the case for the eSATA. That definitely renewed my faith in the build quality of the case, it took a long time to cut through the steel! 
There were a few trials and tribulations to sort out to get this machine stable, but the most INCREDIBLY frustrating problem was a mysterious reboot issue that only showed up once the machine was under heavy load, running 20+ streams of 48khz/24bit audio in Ableton Live while simultaneously running two copies of "CPU Burn-In " and one instance of Prime95 . Sometime between five and fifteen minutes after loading up my Live set and starting the load generator stuff, the machine would suddenly just reboot! Even more troubling - when the machine rebooted, it left the MOTU 828mkII in a state where it was making a highpitched (say 17khz?) tone at a very high volume. That would be a NIGHTMARE situation in a live set - there's no easier way to make a roomfull of enemies than to hurt everyone's ears... Spontaneous reboots are almost always a result of hardware failure, incompatibilities or overheating. With a tiny 1U case, heat was the likely culprit - though I was carefully monitoring heat, and the CPU was never going over 60 degrees, where the manufacturer's documentation said that 65 degrees was the safe point. Still, I replaced the heat sink and thermal compound, going with Arctic Silver 5 and using their ArctiClean product. Once heat was eliminated, I figured the problem must be the RAM. Not knowing the ins and outs of the latest motherboards and RAM, I had purchased 2G of CM2X1024-5400C4 Corsair memory based on price. Now, doing my research, I found that the RAM was not listed in the hardware compatibility list for my motherboard! Biting the bullet, I found a deal on the CM2X1024-6400C4, and promised myself that if it worked, I'd bounce the 5400C4's on Craigslist. Installing the 6400's, I saw a measurable increase in performance, but the problem persisted. If it's not heat or RAM, it must be the power supply. The power supply that I initially bought with the case was a 140w, which theoretically should have been enough, but CPUs draw a lot more juice when they're under load, so it was a logical conclusion. I picked up a 420w rack supply on eBay and installed it, but the reboots continued! Tearing my hair out, I reinstalled the OS several times, hoping to find a hardware or driver combo that wouldn't crash. Each time, I had to contact Ableton again to get them to activate Live for me, which I'm sure they hated. :) There's no way around it - it's got to be a hardware issue. Finally I broke down and blamed the motherboard. I hit the internet again and researched for a week, and decided on the ABIT NF-M2 nView as an acceptable replacement. A few days of the usual driver headaches and problems later, it's up and running. I ran it overnight without issues, and patted myself on the back at having finally solved the problem once and for all... but then it rebooted! Finally, having replaced pretty much every possible component, I decided to try tackling the problem from another angle - what if it's not something in the computer? It would be horrid if for some reason it was the MOTU, so I started by removing the Behringer BCF2000 from the mix. And what do you know, THE MACHINE BECAME STABLE.
Holy crap, colour me UNIMPRESSED. After literally weeks of pulling my hair out, replacing every component in the system, and ending up with pretty much an entire second computer in spare parts, the reboots boiled down to a FRICKIN' USB DEVICE?!? Since when can a USB device take down a whole machine?!? oh. right. windows. Jeeeeeesus. The things you take for granted in the Linux world. I've since switched to using the BCF2000 in MIDI-only mode, using a MIDIMAN 2x2 USB interface. M-Audio makes a fine USB MIDI interface, it's a shame their USB audio interfaces suck so much. 
ANYWAY, SHE'S DONE! As you can see, she's pretty packed - I've already had her out in battle, playing a show in Reno, Nevada. She held up admirably, and I look forward to carrying her into battle many times to come.
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-Adam