I made myself a WHS system. Using a full tower case i had (Bought from here) with a tagan 580W psu (Overkill, but it was a spare)
Rest of the specs were bought as cheap as possible. I even went B Grade to save even more on the mobo.
OcUK 1GB (2x512MB) PC2-6400C5 800MHz DDR2 Dual Channel Kit - £13.99
AMD Athlon 64 X2 Dual Core 4000+ 2.10GHz (Socket AM2) - Retail - £34.99
**B-Grade** Asus M2N4-SLI (Socket AM2) PCI-Express DDR2 Motherboard £24.99
Western Digital Caviar SE16 500GB SATA-II 16MB Cache - OEM (WD5000AAKS) 1 £55.99
Power isnt important. Its now got 3x500gb drives in it. You want to make your system drive the biggest drive you have. WHS automatically partitions 20gb for the system, and the rest is storage. As you can see, it was only £75 for the mobo, cpu and ram. I used a PCI graphics card i had spare to set it up. It was done on the cheap.