I currently have a Sound Blaster Audigy with my Guitar hooked straight through my line in port. I can record with 6 milisecond delay, and the quality is exceptional.

Would I get a cleaner tone with a USB Interface such as Line 6, etc...?

sweet tit licking son of mary and joseph, if you looked for a minute on this forum you would have the answer, threads like these need to ****ing end
5150 III 100W, Mesa 4x12 Cab, Framus Cobra Cab, M-Audio Profire 610, ISA One Pre, SC607-b, Equator D5's, Countryman Type 85 Di box, Radial JCR, Superior Drummer, SSD 4 Ex, TS9, NS-2 and the list goes on and on
^ I agree.....so many repeated posts every day..........

Anyways, if you're happy with the sound quality and the input types work for you, stick with it. If you have cash to burn and want more features as well as improved sound quality, then by all means, go buy an audio interface.