I am wanting to know if doing the pulley bit will give me much gain for the money.
So far I have K&N FIPK, Hypertech, Gibson Supertruck cat back on order.

Rocks

 

If you do both the under-drive and the electric fan mod, you'll be in good shape, that'll free up 20 hp right there. Other things with parasitic drag are your headers, the factory y-pipe, catalytic converters ... all of these things cost significant amounts of money to replace, but their finer points don't really start to show through unless you get the other parts. The underdrive pullies alone will do something, but them and the electric fan mod will really help you out.
Myself, I'd say it was worth it, if you're purely looking for drag abilities. However, if you were actually looking to be able to corner as well, you might look into Anti-Sway bars for $150, a new suspension with a lift or a drop ... there's lot of ways to sink money into this thing, it's a big money pit.

And we all love our money pits, don't we fellas?

 

Rocks

I don't know what motor you have, but you will gain about 13 hp on the 4.6 with ASP underdrive pulleys. The good thing about pulleys is +13 hp all the time at any throttle position. I have them, and I like them. As was mentioned, electric fans will also help you pick up some more power as well as some better air conditioning in stop and go traffic - Derale makes a good one.

 

You'll gain a few HP but how man yonly a dyno will tell.