Your gains should be ok on on just heads and intakes alone, b8t I do not see why you would not want to put a cam in. If you are worried about the idle quality just have the vendor match you a set of heads to the cam. The heads and intake will also you run you more if buy with out the cam. Super Six Motor Sports, Delk, and RPM mustangs all sell Head/Intake/Cam Packages and can range any where from 2200-4000 in price! That is for all new/Reconditioned parts. You can also cruise the boards and be patient likeI was and just buy them peice by peice from some one who is selling them. I saved a ton by doing that. To install can be anywhere from 1000-2500 for a fair price. I would say the gain depend on the package and car, but I have seen a head intake cam packege with a single port average about 190 @ the wheels or more and a 260 @ the wheels on a split port car <kirk or silver stang> Any way it really depends on the aplicationa dn year of car.
What ever you do make sure you plan for the future and if you are going to turbo or SC make sure you do the bottom end forged too. Plan your build towards your ultimate goal and it you will save tons of money in the end!
Good luck man!!!