crash course on hardware: work out your needs vs budget. Do you really need pristine D/A conversion when performing in a noisy room with bad acoustics? Perhaps the money is better spent elsewhere, like on a good mic with better acoustic response (that'll last you infinitely longer than your digital interface). That said, avoid Behringer and most of these low end guitar-musician designed devices. unless you really don't give a crap and aren't intending to in the future.
Try and get a firewire interface. RME is well recommended, but pricey. Otherwise you can't go wrong with m-audio - which is why every low end user has at least something m-audio. Edirol, Tascam, MOTU e.t.c. all good.
What ever it is, if on a mac, make sure it's core-audio compatible. I can't give you any linux advice i'm afraid, as I haven't got the faintest here, although I REALLY want to get into linux audio.
Dual 1.8 IBM G5: Mac OSX 10.4.11 -- Asus eeePC 701: Pure:Dyne / eeeXubuntu GNU/Linux -- myspace.com/thearifd