A tune and CAI for starters. Not really bang for your buck but made my old v6 feel 100x better. Also some 3.73's in the rear made it feel 500x better.
There's not much you can do on a budget unfortunately unless someone else wants to chime in... Ford made these cars pretty well from the factory in terms of stock performance parts. I guess you could remove the air silencer in the intake and drop a K&N filter in there but it would almost cost as much as a CAI kit.
2003 Black GT - SLP LM1, O/R Xpipe, SR CAI, Bama Tuned, MGW Short throw shifter