[TJR]

In my opinion, I wouldn't spend any money on that stuff if your going to keep the stock heads. Here's my guess for the amount of horsepower you'll pick up with each mod:

Cobra intake- 5-10 rwhp
70mm tb- maybe 1 rwhp, but it could hurt performance.
mass air- 3-5 rwhp
fuel injectors- most likely hurt performance on a stock engine.
fuel pump- 0 rwhp
chip (off the shelf)- waste of money, break out a timing light instead.
custom chip- maybe 1-3 rwhp, wait until you have a lot of mods.

I'd put a different gear (3.55-4.10 depending on transmission, future mods, and miles driven on the street) in the rear end. I've done the bolt-on route, and the gear made the most noticeable gain when compared to all other simple mods (This includes everything on your list, and then some).

If you go with the nitrous though, you'll need to make some of those fuel upgrades, but I wouldn't at this point in time.