Skip the catback, they don't benefit you until some crazy hp number... I think it was 400whp or something.

To go with a 3.4 on a gen 3 I would do headers (Speed daddys are $200 shipped), Autolite 104 plugs, 180* tstat and a cold air intake (you can make one for $40 if you want)

The following is from ZZPerforamnce.com (Don't rely on ZZP, but this is legit info)

97-03 Grand Prix & 04+ supercharged Impala & Monte Carlo SS
3.5 pulley totally stock car
3.4 pulley Colder Tstat & spark plugs, downpipe, Cold air intake (still could have KR)
3.3 pulley all of the above plus a VS cam, SS intercooler, or headers
3.2 pulley 3.4 mods and a XP cam
3.0 pulley 3.4 mods and a fullsize intercooler or an SS ic and a VS cam.
Anything smaller than a 3.0 pulley will require a camshaft, headers, intercooler & injectors.



04-05 GTP or Comp G & 06 GT supercharged
3.8 pulley totally stock car
3.6 pulley Colder Tstat & spark plugs, downpipe, Cold air intake (still could have KR)
3.4 pulley all of the above plus a VS cam, or SS intercooler
3.3 pulley 3.4 mods and a XP cam
3.2 pulley 3.4 mods and a fullsize intercooler or an SS ic and a VS cam.
Anything smaller than a 3.0 pulley will require a camshaft, headers, intercooler & injectors.