Playing FO3 on the PC, I recently got the urge to turn off the HUD. It was easy enough to find the display settings and change the opacity of the HUD to 0, effectively making it invisible.

Really cool - except that I can't aim very well, thanks to the funny angle the guns are at.

So is there a way to shut off the compass, while keeping that aiming thingy?

Not to my knowledge. However Im sure there's a mod that may fix this. Try asking in the mods section..

Thanks for the reply.

What about simply changing the color of the "enemy" pips from red to green (so they match the "friendly" pips on the compass)?

I'm just trying to see if there's a way I can do this myself via some text-file editing...

I use the http://www.fallout3nexus.com/downloads/file.php?id=6938 mod with a "disabled" HUD and it works like a charm. There are two things which are a bit annoying about it (first: objects not "lighting up" when you have them in target, thus making it a bit of a guesswork; second: locks, computers and so on not giving any indication about the needed lock level anywhere but in the HUD - and you can forget about visually distinguishing good locks from bad ones ...)

Simple. You need Darn's DarNified UI F3 - http://www.gamesas.com/index.php?/topic/1121454-wipzbeta-darnified-ui-f3/

Or you could try this:
http://fallout3nexus.com/downloads/file.php?id=136
It's less likely to crash your game.