Hi
Just changing the muffler should be good for about 2-3 hp, cheapest is to just find a suitable muffler replacement and get a shop to weld it on, you will save heaps $$$ over the cost of a so called "bolt on" performance muffler, lukey are not too bad, less of a sloppy fart sound the better.
Next best mod would be to change the catalytic converter over to a straight thru cat, that should be around similar HP gain as above, If you were thinking of running a straight thru joiner pipe you will have problems especially as the 2.4 has numerous O2 sensors and you will get the check engine light all excited by coming on. You can stop this happening by tricking the computer by using a defouler or O2 sensor spacer, but then you will be illegal so best to use the high flow cat instead of a Joiner pipe
If you do just the catalytic converter and muffler, these are 2 of the major restrictions in any exhaust system, so its a good start!
The re-flash is probably the cheapest bang for buck way to go, but you would need to do some more research if the end result justifies the cost involved, if you had a turbo/ralliart/evo then the gain from doing this may be as much as 30HP, but as its a stock 2.4lt motor, you would have to weigh it up for any small gain you may get, unless you plan on spending mega bucks on engine mods, as i said more research.
Then maybe look at air-box/ intake mods, thats another story.
The 2.4 lt will be quick anyway with just the above few mods, a lot quicker than my CH 2.0lt, sighhhh, that's for sure, just remember squeezing more HP out of the engine will come at some cost, both $$$$ and fuel economy wise.
Cheers