I have a Dell SC440 with the following config so far:
-2.16GHz Dual Core Xeon
-3GB ECC RAM
-512MB Nvidia 8500GT Graphics Card
-Griffin iMic
-Linksys Wireless N PCIe card (which instantly is recognized as an Airport card)
-250GB 7200rpm System Disk
-2x500GB Samsung 7200rpm SATA drives running in Software RAID0 (Risky I know)
-500GB Iomega USB2 external drive with weekly cron to backup my iTunes folder
-320GB WD MyBook with Time Machine to backup my /Users folder.
-Leopard 10.5.4
-Apple Labelled chassis
It runs nicely, I tried it with ubuntu first, but DAAP (iTunes sharing), uPnP (for xbox and ps3) wouldnt work, so it defeated the purpose of it.
The only thing that doesnt work is CI/QE but I think that is because the graphics card I installed isn't working properly, it was old.
It serves my MacBook, my Mac Mini G4 (which is at my sister's), my xbox360, Airport Express and my sister's PS3.
It does this pretty nicely, but I'd like to get 1TB disks now they are going down in price, replace the DVD drive with a 2x5.25 to 3x3.5 inch bay and then get some more RAM and a new graphics card, then it'll be more a workstation/server.
All in all it cost about £500 including Leopard.