For under $600 there are really only 2 real options
this:
Nissan 370Z Z34 09-11 Top Speed Pro-1 70mm Catback Exhaust System Systems Race | eBay
or this:
CNT Racing Catback Exhaust - CB-370Z - Nissan - 370Z - From: 2009 - To: 2011 | NextGenTuning.com
both have decent reviews on this forum. and i think u will find that most people like the quality and sound of these 2 systems generally. u can find videos of both around youtube.
if u want a deeper tone, then a megan exhaust is the opposite of that. defiantely the highest pitched exhaust i've heard on these cars yet