if you think you may switch OSs eventually, you probably want to pick a DAW that's available for both Mac and Windows (ie Cubase, Reason), so you don't have a learning curve when you do the switch.
As far as an audio interface goes, you'll probably want one with MIDI (in case you have older keyboards), Hi-Z inputs (for guitar) and as many inputs you think you'll need. Fishy mentioned FireWire is better, but I've never used anything FireWire, all I can say is that USB isn't bad at all. With ASIO drivers, you can get really low latency and whatever you do DO NOOOOOOOOT get the M-Audio Fast Track Ultra, it's utter !@#$.