I'm debating on what I should do next. My choices are:

Voltage Booster (about $70)
3.25 pulley - (about $60 + smaller belt $40 = 100)
Scan gauge (about $300)
Electric water pump (about $300 plus smaller belt $40)
SLP headers (about $700 plus install)

Any suggestions?

Voltage Booster (about $70) - eh... do you really need this?
3.25 pulley - (about $60 + smaller belt $40 = 100) - you don't have near the mods to support this small of a pulley
Scan gauge (about $300) - very good idea
Electric water pump (about $300 plus smaller belt $40) - waste of money IMO
SLP headers (about $700 plus install) - very good idea

EXACTLY how I would have classified the options.

I'll go with the scan gauge as well. Then you know what'* going on before you start adding more mods. Have you looked at pacesetter headers? Those come to be about $500. Forget the pulley, for now. The other stuff is not needed, for a while.

Yeah, I think the scangauge is the best bet too. I saw that someone else cut out the cigerette lighter and it fit nicely in the 99.

I haven't heard many good things about Pacesetters.

So then, next mod = scan gauge, then headers, then pulley?

Before I'd drop the pulley size anymore than a 3.4 (after the scan guage of course) I'd say headers, then rockers, then pulley down below the 3.4 as the scan guage allows.

like everyone else said you're going to need rockers or a cam to get down to a 3.25 without blowing it up. Go with the headers but the scan gauge is always smart to have.

Another option are PEMs, much lower price, they bolt right in without modifications. Rockers, Springs, and pushrods are a good idea if you really want to open that engine up. Forget the pulley for now. If your going to drop it, only go to 3.4" and do that after the scan gauge. To go lower then 3.4" you will need to allow the engine to breath. The exhaust mods, and rockers will do this. Changing your manifolds will take 4 to 5 hours, and a LOT of frustration. You'll carve up your arms, and will not have a great day. Rockers/springs/pushrods will take 5 to 6 hours. Is not as hard as you may think. Go slow and it will go well. As long as you dont drop a keeper down in to the engine.

PEM'* are nice, but after doing that install around this time last year, and now about to buy a set of headers used locally I'm kicking myself for not just getting headers in the first place.