[SCOTTLT79]

If I were you, I would pick up the new issue of 5.0 Mustang and Super Fords. Its the august issue. The cover article is "Fast Start". It has a list of the most important first mods to put on the stang and a few paragraphs explaining each one. It is a very informative issue, especially for beginners like some of us. It is helping me a lot in making my decisions to start the modification process.
Many of them have been mentioned here.
Here is a quick list but you can pick up the issue for an explaination on each.

Rear End Gears
Pulleys
Cold-Air Intake
Mass Air Meter/Throttle Body
Exhaust H-Pipe and Cat-Back (Headers if you have the money)
And then a stretch is to go for the Heads, Intake and Cam.

I hope that this helps a little, but again, the article is very detailed and product specific.

Scott