To start this should give you a pretty good idea of what you currently have down there:
Pipe size is 2.25.
You will find plenty of threads in the Forum that speak about exhaust modifications, but since I read quite a few of them, I'll give you a quick recap:
Some guys replaced the last muffler with a Remus, Brabus , AMG, etc and they were happy with that.
Some, like me, removed the resonator and replaced the muffler with something less expensive than the aboves, Magnaflow, Flowmaster, etc and are happy with that new sound.
Some removed both (resonator and muffler) and ran straight pipes for an even louder setup.
All sizes from 2.25 to 3 have been used in different setups with good results (to a personal liking degree).
Like I said, a quick search will produce a lot of information on this subject.
IMO the R129 is a classy car, but the V8 is too muffled from factory and needs to be modified. Not too loud, but it needs to sound like the sport car it also is. I'm very happy with my current setup.