there's a good amount of bolt-on options for the 3.4, it really depends on how serious you want to get with the car.
the abridged version:
cold air intake from slp
some form of cat-back exhaust (i like magnaflow)
ignition upgrades like wires, plugs, and coils
you can get an underdrive pulley from ASP
im looking into an MSD system
comp cams well...cam
your general port and polish
if you are hard core you can get the 1.8 rockers from slp
and then there's stuff like rear-axle swaps and weight reductions...