Bigger iTunes Storage Server Help

Ok i managed to buy on eBay tonight a HP XW4400 workstation in which once i have it in my clutches max the storage on it. But my dilemma is this :

I own :
1 x Mac Mini Server i7 with a 1Tb Hard Drive and 8Gb Memory
2 x Apple TV Gen 3
1 x Apple Airport Express (Connected to Sky Router)
1 x Apple Airport Extreme
1 x Apple Time Capsule

My Objective :

So the Workstation i bought tonight i want to use in conjunction with the Apple Mac Mini Server in which is being the iTunes server for the Apple TVs.

My Problem :

Not knowing what to use for an operating system for the Workstation and once i have decided will i be able use the Mac Mini Server to use the workstation to get the iTunes data i will store on it.

The Ideas for the Operating System im not sure on using :

Microsoft Server 2003 or 2008

or do i use something else ?