From what I saw when I was doing my transmission flush you can simply "UNBOLT/BOLT on" an exhaust on our cars. The only one that I think would work that way would be the UUC exhaust system because it''s a COMPLETE swap (hence the high price). I had the UUC exhaust on my last car (1996 328is) it's very cool sounding gives you that mean very sporty sound. If you want something a tad bit more glassy yet aggressive, I would look into the AA and the Rogue IKON. But do read up on the thread that BMWLotAttendant posted. And if you would please send me a private message, I'll give you a BUNCH of links for comparing different exhausts.
One piece of advice. Last year when I was doing some serious research on exhaust I looked into EVERY details of all the ones out there and I read some heavy negative feedbacks on the remus (mainly rusting issues).