Building a computer yourself really isn't all that difficult.
You could probably make the same thing mynameisscott wants for about 500 less, if you build it yourself.
I myself made a recent investment because I realized I was bringing more and more projects home from the studio.But if all you have is 700 to spend, I'll tell you right now, you're going to get stuck in some places.
To stay on track:
1) A good portion of registered memory (1gb works well)
2) SATA aka Serial ATA hard drive (80 gb should work well)
3) Power supply 400-450watts
Others to keep in mind:
Prosumer sound card (within your budget)
Quiet case
Heatsink for your CPU
Here's an example of my recent DAW to give you an example of what I mean:
2 AMD MP CPUs at 1.8 gigs each
2 Sticks of 1gb Samsung registered memory
2 200 gig IDE Harddrives
A skyhawk quiet case (Rackmountable)
2 Thermaltake volcano heaksinks for the CPUs
Nvidia graphics card
Pro tools HD 3 accel sound card (ignore this one)
Minus the sound card I have, this set up would cost you around $1300-$1500. However, with the soundcard, add another $14000 dollars (yes, you read it right)
pricewatch.com is a good site for parts.