So all you need is computer-only right? I'm assuming you won't be needing monitor/keyboard/speakers/mouse/media-card readers/ floppy/printer.
Let's see for that type of server. You should probably get a dual-core AMD. Maybe dual-core opteron. i don't know much about the motherboards. You would need ecc ram for the motherboard. Maybe get 2 raptors + half- a terabyte (2x250)GB SATA3 Hard drives in RAID.
First got a computer 4 years ago. Windows ME. Got second hand computers with different operating systems from my family members to play around with...
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ASPIRE ATXB4KLW-AL Antec 400W
ASUS A8N-SLI Deluxe
AMD 3700+ San Diego
Corsair 1GB 2x512
250GB Hard Drive
Sound Blaster Audigy
Windows XP HOME W/SP2