OK, for your needs (live guitar, bass, vox) I’d look into the http://www.sweetwater.com/store/detail/FirewireSolo/ (I’m not recommending Sweetwater, it’s just where I found the info). It really seems to be made for the singer/songwriter/player…and has ASIO compatibility.

As for external sound devices. In this day and age, I would really recommend getting a small MIDI keyboard and buying some VSTis. You can go soooo far with it now that anything really external is so much more of a hassle than it is a help. Depending on your budget you can get something like NI Kontakt and a small keyboard added to your EZDrummer VSTi and be golden. The only limit would be on your end of things (playing, songwriting, etc.) and you wouldn’t be held back by technology.