If your looking for a Buget setup then a RaceLand 4-2-1 S/S Manifold and then a Cat back Ashley Competion System with either 3' or 4' tailpipe good performance and very loud check out
www.rallydesign.co.uk for ashley exhausts and
www.kam-racing.com for raceland stuff. RaceLand manifold is about £130ish then about the same for the ashley system.
On a higher budget GMC 4-1 manifold and then Scropion or Piper cat back exhaust not a loud as the ashley but will be s/s rather than m/s (not that it really matters) expect to pay about £250+ for the manifold and about the same for the cat back exhausts
If money is totally no object then go for a OMP system from YBracing on eBay - very light and very good performer, it will however replace the cat (non MoT friendly) and expect to pay £300+ just for the exhaust
Magnex and Janspeed will ruffly sound the same Piper and Scorpion will be louder depending on tailpipe choice and Ashley and OMP will be very loud.
Depends what you want and how much you got to play with