Question 1: yes
Question 2: likely not

Yes you could use a USB interface which probably has better converters than the onboard sound card in the laptop. I would bet that it does. However, will you hear a "useable" difference. I doubt it....even with a really nice high dollar interface. Most of the converters are fairly decent and the audio amp section of the card in a lap top is pretty nice too. My laptop, an old one, sure sounds good with my cans. So I'm gonna say no.... I don't think there will be enough of a difference to justify the cost.

There is a better, easier, and cheaper way to improve the quality of the music. And you already likely have it.

Getting away from the synth sounds would be the best option.

In BB, you have a variety of styles....thousands actually. Many of them are using real tracks but the vast majority use MIDI and therefore synth patches (not samples) to create the sounds you hear. As a result, the sounds are less than stellar.

No matter what basic synth you use, (Coyote, TTS, Wavetable) it will retain that midi stiffness. You can however, take the time and manually replace the default synths with a custom synth as several folks previously mentioned. Something like Kontakt or GPO, etc...This gets you a better sound quality. It's still midi based, although much better sounds, often sampled as opposed to patches.

Taking the project into Real Band and using all real tracks will take it to an even higher level of quality IMHO. That gets you totally away from the synthesized sounds and using real, sampled, instrument sounds. I think with that setup, you will hear a difference. Not necessarily in sound quality but in the sound quality of the playing. It won't sound so fake, and so "midi".

Simply save the projects out of Real Band..... or better yet, export them as Wave files and arrange them in your favorite Music player in the laptop. Set them up in play lists and they will load instantly, and you have less of a chance of Murphy, and his laws, popping up in the middle of a performance.

Some folks like to run the music out of Real Band or BiaB saying they like to be able to do this or that in a show....OK, whatever....... to me, it doesn't matter. Out of RB/BB or Windows Media Player or any other music player.... the tracks will sound the exact same way to the audience. It's easier to use a simple player. But I guess some folks like that BB will run the words on the screen.

You can find my music at:
www.herbhartley.com
Add nothing that adds nothing to the music.
You can make excuses or you can make progress but not both.

The magic you are looking for is in the work you are avoiding.