First question: have you ever built a PC before? Do you know how to match motherboard specs with parts? The first thing that is gonna dictate your purchase is the soundcard that you're gonna be using. Onboard soundcards are not suitable for production. If you already have an aftermarket soundcard, It may only be compatible with certain chipsets/processors.
I don't post much, but I feel like I can add something valuable to the discussion. I've built more than 2 DAWs recently, and here is what I would do if I were in the market to build a DAW from the ground up for personal use right now. (Sorry for not answering the question directly)
#1. Sorry if someone already mentioned this, and I missed it. I HIGHLY RECOMMEND AT LEAST A RAID 1. should be a mandated standard on any serious DAW. RAID 10 would be nice, but the price would seem to dictate otherwise. RAID 1 in basic terms is a setup of 2 hard drives which are mirrored, they both store the same exact thing. If a hard drive crashes at any given moment, an exact copy exists inside the machine, and you can rebuild the RAID set from the existing hard drive (with the touch of a key), and wont lose ANY DATA. If you don't think this is important, YOU WILL someday, and this post will come back to haunt you at night. Trust me. Dont give me "back up! back up!" yadda yadda, were here to write music, not back up. Don't be intimidated by setting up a RAID array, it's not that much harder than installing windows. If you've ever lost any artistic works to head crash, you will agree that this is absolutely necessary.
#2. I highly recommend (from personal experience) is a nice rackmount chassis. I have this one in/on my current DAW:
http://www.newegg.com/Product/Product.a ... -_-Product And mine came with a nice ANTEC 550W p/s. This think is built like a fockin tank, all steel. I ran live visuals at an outdoor party all weekend in the dead of summer. The chassis took all kinds of abuse and loved it. I had to completely disassemble it and give it a bath. No problem. It's big and heavy as shit tho, (40+ pounds, however many stone where you live) but virtually indestructible. I have a nice studio workstation/desk (which cost more than most synths i own, and one of the best investments ive ever made in production equipment, as a side note) - rackmounted was extremely important to me. Super nice design - everything is modular, including case fans, everything fits with plenty of room, and its lockable. Not lame ass plastic door lockable, you get the drift. As a fringe benefit, its a lot harder to steal than a standard tower chassis.
#3. Def not turning this into apple vs mac thread. But I'm told by a mac guy that you can buy apple compatible pc hardware (somehow). Maybe someone else knows the details on this. If i were to do it all over again, I would get apple approved hardware, so i can have a native dual boot apple/windows 7 situation. I am a windows man, but like everyone says, why limit yourself. That may not be necessary, just a quick google search turns up this:
http://lifehacker.com/321913/build-a-ha ... -under-800
couple things i wanna point out here. If you get the above mentioned case, its about as quiet as a jet engine. People say to buy the 'silent' type fans, which are about 4-10x more expensive than your standard 'loud' fan. I know I'm gonna sound like a jerk here, but my 12+ years IT experience tells me that they're total shit. They'll fail in about a year, or two at max, the same amount of time it takes for the 5 dolla chinese junk to fail. If you have a budget for quiet fans, by all means do it. I have to live with noisy fans, although my ASUS motherboard lets me lower the RPM of the chassis and processor fans, making it MUCH quieter. Of course when I think of building a DAW, I'm thinking lots of outboard gear, thus lots of heat. My room is packed with shit --- multiple synths, mixers, stringed instruments, books, spare parts, psychedelic artwork, convolution foam - you get the picture. One of those 'quiet' types of cases is just gonna cause my stuff to get cooked. It depends entirely on your intended use

If getting Silent type chassis - go for the ANTEC Sonata (if they still make em).
Thumbs up to ASUS motherboards - good advice.
Also, as everyone said, get more RAM later if you have to. I would say 4GB is minimum these days. I've got 2 GB, and have crashed out ableton on several occasions. I'm not sure how everyone else is doing things, but in Ableton 8, I am constantly freezing/flattening tracks, then assigning my samples to RAM. Couple this with software like Toontrack EZDrummer, with expansions, and Spectrasonics Omnisphere. Now have a couple of instances of each (for example, sometime I use two copies of EZD, one track with a standard kit for hat work mostly, then another copy with the latin percussion for that sorta thing) Just those two instances are more than 1.5 GB of data stored to RAM, without any other tracks. If you're not assigning samples to RAM, then you're quickly gonna bog down the hard drive bus.
There is a way to determine exactly what size power supply you'll need. Simple Addition. All peripherls have a power consumption rating - my old AMD processor was 18 watts. You find all the power ratings for all the peripherals you intend on using, and add them. Get a power supply rated higher than this number, and you'll be fine. Don't ever get a cheap power supply. Somehow my CompUSA branded power supply shorted about 10 years ago, and actually CAUGHT FIRE TO MY MOTHERBOARD. Not a joke or an embellishment. Don't skimp here. (side note, maybe never buy anything with USA in the brand name, yet manufactured in China. Only buy if actually made in the USA... lol)
So yeah, give me some more info, like your experience, what soundcard you'll be using, you're intended purposes. Maybe I'll write a book next post.