For best bang for buck this is the order I'd do stuff in...
1. Dinan 3.62 Diff
2. Crank Pulley
3. ECU remap
4. Down Pipe with Sports Cats
5. Catback Exhaust
Of course if you were going to do all of the above, if possible, it would be a good idea to get the ECU remapped for the entire setup (i.e. taking into account all of the modifications, particularly the down pipe)...
If I had to choose between catback exhaust or down pipe because of budget and my criteria was performance (not sound), the down pipe would be the way to go and they both cost a similar amount (assuming AA brings out the sports cat version soon)...