However, your synth will sound pretty much the same (GS wavetable)

An external soundcard may improve your audio recording (actual sounds) andthe quality of the sound at playback.. but your MIDI sounds will sound the same pretty much. Same sounds played thru better convertors..

Example- realtracks are audio, they may sound a 'little' better with new external soundcard.
A lot of BiaB tracks are midi, which is just instructional data. Those tracks play thru a synth to generate the actual sound (which is then sent to the soundcard output so you can hear it). WIth an external USB device you'll most likely be using the same synth to generate those sounds. SO, again, you may hear a 'little' difference in the sound at playback because of the better convertors, but it will still be the same cheesy (sorry) wavetable synth generating the sound.

External devices like the M-Audio do not usually come with the midi synth sounds (the GM part of your question).
Actually a LOT of internal soundcards don't come with sounds either, which is why there is the MSwavetable synth to begin with. They just use that one for free.
You should try the VSC synth that came with BiaB or maybe try the Forte synth for 30 days free trial (downloadable from this site) I think THAT will make a bigger difference for less $$ 'specially when the VSC is already owned, so free, and there are others out there with much better sounds.

Audio interfaces and soundcards are terms often used interchangably but the midi/audio definitions are most often misunderstood.
An audio interface is usually a step up from a basic soundcard in that it has better convertors (to change the sound from analog to digital and back) and also more convenient connections and inputs. Some include preamps that are much better than a 'soundcard' usually.

What instrument you trying to connect?
Maybe an audio interface will help you with those connections and recordings. But if you are using wavetable as the synth I just wanted to point out you can improve that part right away before investing in a soundcard.

I do not work here, but the benefits are still awesome
Make your sound your own!