[KillboyPowerhead]

Best bang-for-your-buck mods might include:

Cold air intake;
Jacfab's Laminator FX (perhaps not as effective as a ported/modified factory manifold but a lot cheaper);
Jacfab's Extendolator (spacer between plenum and manifold);
Removing the secondary cat on the factory downpipes;
Tune.

From there it all comes down to how much you want to spend:

Ported factory manifold;
80mm throttle body;
High-flow cats (if you removed the secondary cats on the factory pipes I suspect these wouldn't add much benefit);
Cams (Mace Engineering makes these; I have them but haven't installed them - plan to fairly soon!).
Rear end swap (such as going to the 1LE rear end for 3.91 gears).
E85 kit.

Intake + exhaust + tune will get you around 300 WHP (stock is ~250); E85 will get you another 15 HP I believe; cams can get you another 40 or so with proper tuning (seems to be a lot of uncertainty about how much power these cams can make as some tuners don't play with the variable valve timing and allow the engine to spin past redline - guess I'll find out for myself after I install them). In any case, full bolt-ons including cams will get you somewhere in the vicinity of 400+ HP/~320 lb/ft torque at the crank which ain't too shabby for an NA 3.6 V6! With 3.91 gears on top of this you're looking at a 0-60 in ~5.0 seconds, also not bad for a fat ass car.