nater9 provided a great link that I am reading up on
and this page was interesting to me
http://www.tweakheadz.com/laboratory/hook_up_your_audio_now.htm
just jacking in a instrument into your line-in of your soundcard is NOT
the way to go for serious recording
that is why for example the M-Audio cards and others have zero-latency monitoring
besides latency another issue is feedback whine
especially with a mic or guitar if you are too close to your speakers
than you will get feedback
for my "fun" recording I use headphones to get around this
that M-Audio 2496 card I linked is actually recommended by that site
(I didnt know that when I linked it - just pure luck
)
at that price point of under a $100 as you stated in original post
I think that would be the best bet
It is a little older but the drivers by now should be very stable
so my official answer to your post
is this card the M-Audio 2496 for about $80
http://www.google.com/products/catalog?q=m-audio&hl=en&prmd=ivnsr&bav=on.2,or.r_gc.r_pw.&biw=1920&bih=888&wrapid=tlif130979273150710&um=1&ie=UTF-8&tbm=shop&cid=8592815504381270436&sa=X&ei=5tkRTs3aNabW0QGJsa2KDg&ved=0CKcBEPMCMAI#
and read that link that nater9 posted
also if you want to keep costs down
then software wise
I would go with freeware
I like Reaper Free Edition
http://www.freewarefiles.com/REAPER-Last-Freeware-Version_program_43366.html
also i have heard good things about this one though I havent used it
http://www.freewarefiles.com/DarkWave-Studio_program_58483.html
I use Reaper and it is a good way to acquainted with multitrack recording
high end stuff like Fruity Loops and Sony Acid Pro are great
but can be expensive to start with