mckennma is right. I'd suggest something like the following:
1. Two Opteron 2200 series CPUs (e.g. 2210, 2212, 2214, 2216, 2218, 2220, 2222)
2. Dual socket F motherboard with at least 1 PCIe x16 and 1 PCIe x4/x8 or PCI-X 133 slot
2. Heatsinks for the two Opterons
3. 4 1GB or 2 GB sticks of ECC registered DDR2-667
4. WD Raptor 74 GB (740ADFD, 16 MB cache) for the OS
5. 2x WD Raptor 74 GB in RAID 0 for a temp/scratch file (you can also use 150 GB ones or more than two if you need more space.)
6. 4 or some even number greater than 4 7200 rpm SATA HDDs in RAID 10 to serve as the data disks. Remember that RAID 10 halves the data storage capacity of the drives- four 500 GB drives in RAID 10 give you 1000 GB storage space.
7. A PCIe x4/x8 or PCI-X 133 MHz RAID controller card with at least 8 SATA ports. Since you'll probably run Windows, get a hardware one made by 3ware, Areca, or Intel instead of a Promise or HighPoint software unit.
8. Enough of a case to hold the HDDs and the motherboard- this means a big server case.
9. A good power supply of at least 600 watts. If you use a high-end GPU, then add 100 watts.
10. Some sort of PCIe x16 graphics card. You can get a basic one for $50 or so or buy a really nice one for hundreds of dollars. Integrated graphics can't drive a large monitor very well and most won't do DVI, so you will want a GPU.
11. You may want a UPS and line conditioner also.
12. Keyboard, mouse, DVD burners, etc. Add as needed.
That should give you a start.