#1 -
Lower Control Arms. This is a must, you can add all the power mods you want and it won't mean squat when you're wheel-hoppin down the drag strip. Even without other mods,
LCA's will probably shave a good .2 off your quarter time.
#2 - I'd probably get a nice axle-back exhaust. Slightly improved performance, and great sound (and still within your budget).
#3 - I'd consider doing pullies. Probably the best bang for your buck power-wise.
#4 - At this point, you'll probably need some more drag friendly tires, at least in the rear. Some good drag radials are going to run you about $170 a pop.
CAI is defiantely a great mod for the '05 Stangs. The problem though, is that you MUST get a tune after getting a CAI, so once it's all said and done, the thing is going to cost you at
least $500 (flash tuning devices are about $400, and a custom tune is going to run about the same). If you can still afford it, go for it. I personally am going to wait until I have a few other mods though, then get the CAI last, and get a custom tune. You'll have to pay for the tune in the end, but then your car will be running at max performance.