What sort of old PC are you considering for WHS? Also, are you considering WHS v1 (32-bit) or v2 "Vail" (64-bit only and requires a 64-bit CPU)? It does matter.
There can also be some issues configuring drivers for home-built WHS systems. For WHS v1, if you need a driver that WHS can't find, you can usually use XP or Windows Server 2003 (32-bit version for both) drivers. For "Vail" you can use Server 2008/Win7/Vista drivers.
You can also use FreeNAS to build a home server for exactly $0 that can do most anything that WHS does. Your call.
I built a home server to be used as a version control system and build server for my personal programming projects. I also have it as a domain controller and hosts my IIS websites that is linked to my godaddy domain account.
I just needed something small, wasn't planning on doing any HD videos or anything CPU intensive.
Here is what I built..
CPU + Motherboard + VGA Combo:ASUS AT5NM10-I Intel Atom D510 BGA559 Intel NM10 Mini ITX Motherboard/CPU Combo
Memory: Corsair CM2X2048-6400C5 XMS2 2GB PC2-6400 800MHz 240-Pin DDR2 CL5 Desktop Memory Module
Case: MI-008 Tower Black P4 Chassis with 250W Itx Psu+sata Power Supply
Bought it all on Amazon since I had a gift card to use from them... Motherboard has room for 2 memory slots, 1x 2gb worked as single channel, may eventually buy another 2gb
Cost me: $200
Already had a 500GB Hard drive that I used for the OS
It worked with Windows Server 2003, Windows Home Server, had problems with Windows Home Server Vail, crashed once it booted into OS, but CPU is 64bit, installed fine just didn't want to finish setup. I am current running Windows Server 2008 R2 x64