I don't want to start a mac vs PC debate on here, but if you're using such a high quality FireWire audio interface, have you considered a mac with its native FireWire support?
Would you be using it purely for music creation? Do you use a lot of soft synths and therefore require a lot of CPU?
If you're not in need of the fastest spec then I would look at a mac mini, possibly second-hand.
I doubt you will find an off-the-shelf PC with FireWire, so you will need to do a bit of building and configuring I'm afraid. I've not had to buy or build a PC for ten years now so I'm sure someone on here will be able to give you a more accurate picture of the kid of PC you need.
If you get the choice, Windows 7 beats 8.