If you don't know how to or don't want to do any engine mods yourself, then the CAI is the cheapest and quickest. About $50 - $150, depending on what and where you buy.
Getting a set of underdrive pulleys will give you a few more HP as well as increasing your gas mileage a little. I paid $135 for my set of 3 pulleys and a new belt.
Getting a dual exhaust will increae the HP also. This one costs more that the 2 above. A GT take-off exhaust works well, generally about $100 - $150, then factor in labor to put it on. I went to a local mom & pop muffler shop and paid less than $200 for a 2.5in dual exhaust including the FlowMaster mufflers. I had them cut the pipes about 4 inches past the rear 02 sensor and they custom made the pipes from there to the mufflers and over the axle.
You can buy a dual exhaust kit at a lot of different places, but it can cost a pretty penny.
As far as headers go, several people say the short tubes won't make much of a difference. The long tube header make the most power, but then you will have to relocate the cats and get an extension wire harness for the 02 sensors. The extensions cost about $50 a piece and you have 4 of them.
Depending on your local laws, you may not even need the CATS to pass inspection depending on your mileage. So the CATS and 02 sensor relocation may not be necessary, BUT your CEL will be on and throwing codes, UNLESS you install the 02 sensor deletes. They trick your computer into thinking the 02 sensors are working properly, even though they're not installed. These still run about $50 a piece.
__________________ 2008 Mustang GT Premium 5 speed, 3.55 gears, black-on-black, IUP, 18in polished aluminum wheels, FRPP GTA Axle Backs, JLT II CAI w/SCT X-Cal 2. 296HP & 311TQ
"Some people live their entire lifetime and wonder if they ever made a difference to the world. Marines don't have that problem." -- Ronald Reagan | 
