I'm an AMD guy myself, but that is mainly to be anti-establishment

When I built mine a year and a half ago I went the economical route and got a vanilla AMD board (Shuttle brand) that had lots of slots (I can't remember the chipset). I think I wound up with an Athlon 2500XP with 512Mb RAM. This past weekend I finally hit a limit with my RAM so I picked up another 512 stick to put in (that limit was about 40 tracks with TONS of plugins simultaneously active in Cubase). I've actually mixed a 45 minute/24 track live set with a moderate number of plugins without a hiccup.
But, for the sake of discussion, if I were doing it with your budget, I'd go for an Athlon 64 with 2Gb RAM as a basic unit. Get a mother board that does NOT have video built in so you can get a dual output video card to drive 2 monitors. Check out the MOTU website to see if there are any compatibility issues before making your final decision.
You've already got firewire, so make sure you've got some USB 2.0 slots. Buy the largest case you can find with the biggest power supply you can find. I typically have 2 internal hard drives (system drive and data drive), so get the biggest you can afford for both of those. Since I have a Mackie MDR HD recorder, I am able to pull out my removable drive and just pop it into a drive slot in my PC, turn on power, and "voila" I have a 3rd active hard drive. The only thing I need now is a DVD recorder for backups (which you've got already). Since you have to go through the Fireport, you don't need to dedicate a drive bay for this, obviously.
I guess the only other thing I'd throw in would be dual LCD monitors. I'd love to have a set of 17 inchers myself, but 19 inchers would be even sweeter.
Oh, yeah, one other thing. I absolutely refuse to have my studio PC connected to the internet. My OS is a stripped down version of Windows 2000 and I have never had a lockup, slowdown or any issues (well, except for running out of memory last Sunday).
I don't have any specific manufacturer recommendations other than noted above, but I have had great success with mine. I already had a video card, CDRW drive, firewire card, and a 30 GB hard drive, so my total cost to construct what I would up with was close to $300 including motherboard, processor, memory, 120 Gb data hard drive, and case. I bought everything local so I could have shaved a few $$ by going over the web but I was trying to buy things piece by piece so my wife wouldn't know about it (it was actually 6 months before she noticed!!).
Anyway, good luck. It is a gratifying project once you get it powered up and running and it doesn't really take that long to assemble one.
Darryl.....