There was a thread on this a little while ago.The best bet is to go for the Termignoni assymetrical system.No real power gains,but the price has recently dropped to under a grand.Included in the kit is a pair of air filters and a new CPU.Fitting this wont affect warranty etc.If its more power you are after,then the 2-1-2 systems are the way to go.Expext a price of 2k+!
One of the best looking systems is the LeoVince but the price is silly money.
Casoli do a twin can (996 style) and single can system but both are pretty ugly.
The ugliest of all tho,are the Akropovic and Marving cans,truely hideous.
I found another nice set up while in the 'States made by HPE Tubistyle.No doubt the price will be equal to a small family saloon car!!
There is one light at the end of the tunnel,a guy who posts on this very board,Spyke 748,does a can and exchange front pipe for around 550 quid.Might be worth doing a search to find his details.
Hope this helps.