It all depends on how far you want to go really and how much you want to spent.
1st you need to deside 1) Are you going to change just the back box or go full system?
2) If your changing the whole system are you going to twin box (Centre Silencer) or Striaght through (No in-system Silencer just back box)
3) Are you going to keep the standard Cat, fit a sports Cat (better gas flow but still MoT friendly) or DeCat (No Cat, MoT Failure)
4) Are you going to change the cast iron Exhaust manifold for a Mild or Stainless Steel race inspired 4-2-1 or 4-1 (This means there are 4pipes that go into 2pipes then into 1pipe which bolts to the exhaust) these will greatly improve gas flow into a performance exhaust there improving overall performance of the engine.
Once you have decided these 4things it is clearer on what options are are open to you then its more to do with the bit you can see the tailpipe which most people go on.
I was going to go for a OMP but wanted to keep my cat (or change it for a sports cat) so I couldnt justify the price of the OMP then have it cut to fit a cat'd car