I have recently purchased a G4 cube with cubase and a midiman midisport 2x2 midi interface. I also have a roland mc505 and would like to have some kind of audio now.
I think I've narrowed it down to two choices, there's really not much to choose from yet, when all i can use is either USB or Firewire:

1.MOTU 828 firewire interface
(http://www.motu.com/english/motuaudio/828/body.html)
or
2.M Audio Midiman Audiosport Quatro USB interface with a mixer.
(http://marsmusic.com/store/product_new.jhtml?catid=1474&prodid=13960&skuid=0&deptid=9)

i figure i will probaly eventually need a mixer, and a friend of mine thinks choice 2 is the way to go, but i really like what im hearing about the 828, and im not sure if i should trust usb for audio...

any suggestions?

To be honest I don't know to much about the specific qualities of these two, But if its a choice between a USB device and a Firewire device I have to say go Firewire.

If you just have to have a mixer though you should look at the Event EZ-Bus. Digital mixer, USB audio interface, and Midi control surface all in one.