Hi,
Most (all?) of the higher quality audio interfaces seem to use one of the two firewire protocols (400 mb/s or 800 mb/s, with the former being more common at the moment).
I've also read - and people can correct me if I'm wrong - that although USB 2.0 might be faster than its firewire equivalent, that fact is a bit of a red herring because the speed refers to data travelling in one direction only. I.E., firewire has the ability to pass data in both directions simulataneously; I see this as being important in a multitrack situation where you'll be monitoring (listening to) pre-recorded tracks, while singing/playing along to them.
I don't know what your budget is (that would help others here to make suggestions for you), or if you think that there's ever any possibility of needing more than the ability to record more than two tracks simultaneously; that's worth thinking about.
The other issues that you might want to consider, and which affect price a great deal are: quality of the 'pre's' (preamplifiers), stability of the drivers, and quality/accessibility of technical support (presumably, if the first two criterial here are satisfied, you won't need to consider the third).
I know that RME makes universally acclaimed, reliable/stable, reputedly great sounding audio interfaces, and I'm going for the RME Fireface 400 - capable of recording 4 tracks simultaneously. I'm a little nervous that I might very occasionally want to record more than 4 tracks at a time, but the cost of Fireface 800 means that I can only afford the 400.
I hope that this has helped somewhat.
Good luck.
GVDV