Hi spegeddd, welcome to the forum!
What is the guitar amp that you have?
If you get a decent mic and preamp, miking a guitar amp will generally give a better sound than going in direct. So far I have yet to hear any amp simulator effects that come close to a nice sounding 'real' guitar amp.
Yes, if you are using your standard sound card it will degrade the quality. Standard sound cards are not meant for serious audio recording.
It sounds like there are three different areas here:
New audio interface / preamp
Guitar recording (mic vs. direct in)
Vocal recoding (mic)
What budget did you have in mind? Do you want to stay at around $100 or would you be willing to go more?
-tkr