I build my own, so as far as pre-builds, I can't help you there.

The most important thing, in my book, is that you get at least two large hard drives (120Gig+) - one for the OS, one for audio tracks, and 2 or more Gigs of RAM.

Budget?

Have you considered a mac?

To elaborate on bent and huseph:

If you want to build your own you have to investigate what components are compatible and learn from other builders, consider the processing power you need and the budget you have. That is a lot of work but it can save you money plus you will now all ins and outs of your PC.

Since a stock pre-built PC is hardly good enough for what you want to do, forget about a pre-built unless you buy a pre-built DAW-PC from e.g. ADK. Anyhting else would lead you to a Mac.

For working with audio, have a separate audio drive. When you work with samples a lot, it is good to have another separate drive for that. So in total you would have 3 drives.

Don't buy a notebook/MacBook unless you absolutely need to be mobile.

There are several companies out there who will build you a dedicated audio PC. Of course, it is cheapest to build your own. But if you have a job that doesn't involve this kind of knowledge and your time is at all valuable, it doesn't make sense to research this once every several years. The small companies will be closer to the bleeding edge of technology. The larger ones will have a bigger service network which will usually mean a faster response time. The drawback is that they tend to value stability and are less willing to change chips/components to gain a slight performance edge. I've dealt with Sweetwater - a big company. I've been pleased - but I knew the tradeoffs going in. If you do a search you can probably find a lot of small companies.

New Egg: http://www.newegg.com/?CMP=KNC-overturesmx&ATT=hp

Tigerdirect: http://www.tigerdirect.com/

CDW: http://www.cdw.com/default.aspx

Look into your local mom and pop's, I'd rather drive a few miles and spend a couple extra bucks, than buy online when I can.
Keeps the local economy going, ya know?
And, they're more apt to give you help and advice without breaking the bank, whereas the onlines and chains don't care if they get your business or not.

Motherboard -Asus,Abit,Tyan, Intel. Make sure it is compatible with the interface/software you plan to use. Digidesign would be the main concern. Fortunately, they have a compatibility list on their website.
CPU - Take your pick, Intel or AMD. Just make sure your motherboard supports it. But the best one you can afford.
RAM - Buy the best you can afford. No less than Kingston but try for Crucial or better.
PSU - ThermalTake, Antec has some good quiet Power supplies. Consider an Antec Sonata II case w/PSU for the sake of quiet operation.
Soundcard

Is the M Box the fire wire version? Make sure the motherboard has a TI chip set for its fire wire ports if it has them built in. If not then buy a fire wire card that has the TI chip set.

Make double sure as hueseph said on compatibility with the interface and the mother board. Check their forums for compatibility issues.

There are a couple of fan free video cards that will help lower the noise your computer puts out.

You can purchase an after market cpu cooler that will be quieter than the stock cpu fan that comes with the cpu.

Find a quiet and cool running and fast hard drive. Toms Hardware website has a good drive comparison database to help you find the drive that meets your needs.

Find a DVD/"every other type" drive that doesn't sound like a jet airplane taking off(not an easy task).

Big fans can run at slower speeds which can mean a quieter computer.

I like the antec cases as well I got a p180 quiet but is a little massive and not very portable. I am sure there are other adequate cases as well if you search the manufacturers sites for quiet cases it should help give you options.

There are a lot of power supplies out there. I used an enermax Skimping here can cause problems if you dont get a good stable power supply with plenty of wattage.

I found both new egg and mwave had acceptable pricing.

If you elect to have a multi boot system, place both operating systems on the same hard drive.

Use the other hard drive for audio storage only.

The way it was explained to me, when the operating system writes/reads from the working hard drive and you are recording on to this "same" hard drive, errors can happen.

Here is the link to the thread:
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A few years ago, I bought a gaming machine from an online vendor (www.directron.com) . It had 95% of the specs I was looking for, and was on sale the day I went browsing, scored it for 50% of what a built box would have cost me. I've done a couple of minor upgrades to it, but it still holds its own performance wise against newer machines that friends are using (by their accounts). Worth checking this category out!