Hello Cyril,
I went through the same considerations ... though we have an almost exclusive Mac household here (5 Macs), the building of the Windows machine proved as a good idea: Windows 7 ist quite friendly, and just for raw sample-playing power it's hard to beat on the price. Once you have set up your template on it, you can quite forget about it. I bought a MacMini in the first place as a slave, but it got retired soon (if you want it ... )
BTW every wise man I listened (read) to told: use an i7-9xx processor - they seem to be right
Snow Leopard Server: as far as I know there is nothing to be gained sample-playing wise.
Michael