Whatever you do, don't buy a cheap and cheerful sport cat. They're often cheap because the innards are made of a lower quality metal, which fails years sooner than those with a better design and made with better materials.
You can sometimes get away with a 'budget' decat. But like Pudding says, make sure it's 3" at least. The Relentless pipes get good shouts.
I've got a full Powervalve with one of their (huge) sport cats. It wasn't cheap, but I'm happy that it's one of the best solutions available.