Isn't the M-Audio interface a PCI card? If so, you'd have to buy a very old PowerMac G3, G4, early G5 (produced between 1997-2003) with PCI slots in them. Apple hasn't produced anything with PCI slots for several years now. If you got a current Intel Mac, you'd have to use a different interface (FireWire, most likely).
That said, if you do go Mac:
Notebook: MacBook Core2Duo 2GHz is a good choice at $1300 US. Just make sure to get a FireWire external hard drive to record to. That is true with any notebook, Windows or Mac. Good for audio, but not great if you want to get into multi-media (video, animation) as it has no dedicated video memory.
Desktop: iMac 20" Core2Duo 2.16GHz for $1500 US. Runs cool and quiet and a lovely large wide screen. Plenty of horsepower for professional and prosumer apps.
If you've got the budget, you could go with
the MacPro quad Xeon, but the lowest-end model starts at $2000 US and you'd still have to buy a monitor of some kind. Mostly intended for professionals. Extra RAM on these is very expensive. This would honestly be overkill for most people on the Home Recording forums.