get what looks and sounds good to u..
check www.groupbuycenter.com for some decent deals..
the good brands out there are: apexi....comptech...5zigen<sp?>...tanabe...greddy. ..

here's a deal ive found ....
http://wholesalehyperformance.com/specials.html
400 bucks for 5zigen catback exhaust isnt bad at all