New Mustang Owner

[Hammer]

Best bang for the buck:
Gears, Shifter, Pullies, X\H pipe...

Lower bang for the buck:
new MAF. Nothing on the low end, but you will feel it "breathe" easier on the top...

Control Arms and drag radials:
The 60 foot means EVERYTHING!

If you have money left over:
CAIs-
I have seen TONS of CAI'd GTs (including the densecharger) on the dyno, and I've never seen more than a 2 rwhp gain. To be honest, you can get that much of a difference between runs by doing absolutely nothing to the car. (Just the natural variance in-between different runs, slight change in outside air temp, etc... etc...)

Cat-Backs-
You MIGHT get a "true" 1-2 rwhp gain on a stock car.... but in down to earth respects, you'll only notice the change in exhaust tone...