Originally Posted by EXPLICIT

Exhaust - $1500 - $2000 (plenty of options available)
Ecuflash - $1000 - $1500 (verso or ecutek)
Bolt on turbo ~$1500(VF series/TD05/6 series)
Fuel Pump ~$250 (walboro/bosch)
Intercooler ~$500 - $1500 (sti/aftermarket - top mount/Front mount)
Water spray ~$100 - $600 (sti or diy or aftermarket)

That will give you a very noticeable increase and achieveable within your budget.