[Doggiebox] Audio inputs

Hi- I am totally low-end here but find it is satisfactory to plug a Behringer Eurorack mx602a into my g4's line in. I use a separate mixer (another Behringer ub502) and powered speakers for monitoring. This is cheap (mixers total ~$150) and effective for capturing song ideas. I play guitar and sing and use Doggiebox for drums (aiff output) and Bias-Deck for recording/mixing. I don't use midi at the moment though would like to in future. Can someone give me an idea of what I would gain in sound quality using fire wire or usb audio interfaces? Thanks from a sound newbie- Frank