>>1
$900 USD? No problem.
I built a Q6600-based box with a legit copy of the OS for $1100 CAD.
Get an E4400 proc (or comparable, even an E2xxx will be OK)
1-2GB DDR2 RAM (expensive stuff if you wanna harsh OC an Intel E2160)
Inexpensive mobo - try Gigabyte GA-965P-DS3
Power supply - DO NOT SKIMP HERE. Expect to spend $50-75 on this.
GFX card - right now nVidia is pwning... I'm an impartial fan of the GeForce 8600 GTS. Better than a middle of the road card, and the price is right.
Grab a basic DVDRW and some SATA2 hard drive (160-320GB for your budget, as you deem necessary).
If you need to have a monitor in at that price, kick it to 1GB RAM, get XP instead of Vista, and go for an Intel Pentium Dualcore E2160 (aka core 2 duo with slower bus + smaller L2 cache). Go for a smaller HD, skimp on the case (but GET SOMETHING THAT WILL ALLOW YOU TO SETUP DECENT AIRFLOW, and GET DECENT FANS!!!)
If you need a monitor, grab a cheap ass 19 or 20" Acer LCD - should be around $200 USD if you get one at a good price, if not scale back to a 19 or 17" fullscreen. Keyboard - get something tolerable. But get a decent quality CORDED optical/laser mouse.
And yes, you should be able to buy all of your parts from newegg + build yourself.