Display set to auto should be fine.
I have data push off and anything that pulls on a regular basis is either set to pull only when I request it or on the hour (email only). I run few widgets Power control, Bible app, Weatherbug, calendar, and stick notes. None of which do any real pulling anyways.
I use wifi when wifi is available, it's less of a workload than when 3g is patchy and weak. GPS stays off unless I'm using it for something, and I'll turn it on and off as required.
I don't run a task killer and I have a pretty good habit of popping the top-lock every time I put the phone down (even though I get the slide unlock, I'm used to unlocking). I run a live wallpaper even though some would say it's a battery kill too (not sure how because either way, when the screen is lit, it's lit).
I am a moderate texter, and sometimes use slacker, and make a few phone calls a day, some longer than others. I normally get at least a full day charge on my stock battery. Lesser use, more battery. Heavy use, less battery.
Feel free to spend the money for the extended battery is you feel like you need it, but smart setup and use of the dx means the most for battery conservation. For instance, if I'm sitting next to my netbook, I use the netbook to check FB and email, instead of my DX.
Remember, it takes lots of bites to eat an elephant. The battery is a good one, although there are some bad batteries among us. If the phone overheats, it can kill battery life too. Don't overuse and overtask the processor...it will kill the battery too.
It's unlikely that you'll pull an app that really truly kills your battery. The app might make your phone pull non-stop or something weird...just watch what you install and how it makes your phone act.
Don't be afraid of a battery pull here and there. Remember, its a droid running an android OS. With high tech, comes less battery. Got to pay the piper some how.
good luck