I've been recording bands for around 4 years now and while GarageBand is definitely a very, very powerful recording tool, you are going to have to get a program with a little more "umph".
I just switched to Mac OS so im not sure, but if you could get a program called adobe audition to run on a mac, which im sure is possible, you can replicate what you hear on the singers voice.
What the filter is, is essentially a pitch correcter, only with more gain. To be exact, it's called AutoTune, a developer called VST developed a version for audition which is what we use when we personally record bands (my friend and I)
as for using GarageBand, the previous poster has the right idea, pitch correcter and staying on the specific key is your best bet. you could try to turn the gain up on your vocal track, too.