Pretty good suggestions so far,
I'd agree if your transmission is good, I'd look at gauges up front won't help fuel economy or HP but could save you some cash if it keeps you from overheating your transmission.
Killer Dowel pin is a mixed bag lots of people have never had a problem with it; however, when it goes bad it can really cost a lot to fix. Had mine fixed when I did the gov springs and the timinig which probably saved $100 or so because some of the work is common between the three.
With the stock transmission you aren't going to be able to get very agressive with the HP upgrades(probably have to keep it below 300 or so, there are others on the board that can talk to that better than me).
My guess for impact and cost are (you can probably do a lot of these cheaper by shopping around for better prices):
timing, gov springs, KDP - $400 (installed) piece of mind and setting the timing to about 15. 5 should improve fuel mileage.
gauges - $400 (self install) for piece of mind(this is for a three gauge setup, transmission temp, boost, and pyro)
cam plate - $200 + installation primarily more HP - seems most folks like a $10 fuel plate for all around use. (may be able to install this yourself depending on how mechanically inclined you are, I haven't installed one so I can't really say how difficult it is).
injectors - $600 + installation - primarily more HP but can get better mileage - break point seems to be 215 injectors. They are suppposed to provide about +60HP and a bit better mileage. You may be able to install these youself; however, I haven't so I can't tell you how difficult they are to install; however, from what I've read it seems possible for a do-it-yourselfer(is that a word?).
I'd look at the following later as you become addicted to upgrading your truck
BHAF - $450 (self install). Primarily HP but may help mileage. You may not actually need this until you get beyond 350HP(lots of discussion both ways on the topic), also prices are all over the place, you can get filter systems for a good bit less then $450.
4in Exhaust - $750 + installation. Primarily HP, maybe a little mileage (I personally didn't notice any improvement), and reduced EGTs
Other things to consider:
Summit wiring harness - $50 (self install). The 2nd gen trucks route the power for the headlights through the headlight switch in the cab. As a result, the headlight switch has a tendency to fail (costs about $80 in parts + labor costs to repair). The wiring harness reroutes the power to the headlights through relays which helps reduce the load on the headlamp switch and also results in brighter headlights.
If the truck's steering is vague, you may end up installing a PSC steering box, DSS ($170), a 3rd gen trackbar ($450), and/or other aftermarket trackbar to fix the problem.
Shocks - $70 ea + install. Most folks seem to like Bilsteins for all around use.