Well with a server there are a few specs you should decide on. The main ones being rather its 1u 2u or 4U or desktop and what kind of SCSI drives you want. If you go with 1U you will probably have to have an integrated SCSI.
You probably want to go SCSI for hard drives.
You could get 10,000 rpm's or 15,000 rpms
The best 10,000 rpm as far as access time is the quantum
atlas 10K II
10000rpm, 4.7ms access, 8mb cache, 3.5 lp, a/v rated, 68pin qm318400ty-lw
I would suggest the 18gig version.
If you want the absolutely fastest hardrive get the 15,000 rpm the Seagate ST318451LC 15000RPM SCSI NEW MANUGACTURE WARRANTY It has a 3.9 ms access time This is only in a 18 gig version
If you absolutely need a lot of disc space say 50 or 80gig then you will have to settle for lesser access specs but still get a SCSI
As far as CPU why skimp. Get dual 1 ghz (or the maximum cost effective CPU your motherboard supports). and ram is cheap take a look at pricewatch.com 1 gig ram sticks are 480 to 514 depending on what type you need. and 512's are much cheaper.
So overall I suggest something based on a Supermicro 370DL3 or DLE or similiar board with integrated Ultra 160 and Integrated NIC
with that you need cheap graphic card
2 1 gig CPU
2 10,000 or 15,000 rpm hard drives
500 to 4000 megs of ram
maybe redundant power supplies and another NIC card
I would suggest checking what racksaver has constructed as far as rackmount servers at http://racksaver.com/products/index.asp
They provide a good selection of motherboard and part combinations. It could give you some ideas even if you don't buy from them.