Record With My New Imac Options

[bubblingpeter]

hi there,
last week i tried the new powerbook g4 of a friend of mine, fell in love with that wonderful creature, i decided to try out a program for guitar from native-instruments (guitarrig),i noticed that worked pretty well and with almost no latency, but the input volume was anyway to low, and when i tried to turn it up the guitar went in orrible feedback.
so i just ordered an iMac g5, that i guess should go way better on prestations, but i really don't know if it's going to work the way i want.
do i need an amplifier for my mixer to get the sound into the computer with a good recording level?
I don't know much about the internal soundcard either.
thanx for your opinions.
greets Pietro