If I had 5k and a stock car..
I'd ditch the stock air box, and replace it with cone filters.
Buy a good turbo back exhaust
Buy a wideband
Buy a remappable ECU (AEM EMS, or a PFC)
Spend a long time researching how to tune the above ECU.
Buy a boost controller
And then some dyno time, and tune it yourself.
I know its more than 3 things, but its 5k's worth.
Ben