that some of the basics, has far has best for the money beyond that there are so many combos to run it's hard to answer. a trick flow matched kit is nice and so are afrs, a system max and stage 1 cam.what are rwhp goals that will probally help the answer.
Higher ratio rear diff. gears (best by far).
Shift kit (if automatic).
ASP underdrive crank pulley with short belt AC/smog pump bypass.
Remove air silencer and MAF mesh screen.
72mm SN95 5.0 MAF (if Fox Body).
Advance timing.
Electric fan (if Fox Body).
Subframe connectors (especially Fox Body and all verts)
Also don't forget the bolt-offs i.e. weight reduction.
Personally I plan on going with a trick flow top end kit ($2,400) and then eventually a low boost vortech kit ($2,000) at about 5 psi. That combined withe the correct fuel and spark supporting mods with full exhaust should yield a reliable 400rwhp. Of course u could substitue explorer heads/intake for cheaper but I like the TF top end.
Another good option for 400rwhp is a B&G turbo kit on a stock engine.
Best bang for the buck when starting out is the rear gear upgrade. But only if your ready to sacrifice a little drivability for some quicker acceleration