HI Harg, welcome back! As far as realistic mountains go, I use the method outlined by Ascension in his Atlas style map (i.e. generate clouds, copy and paste them where you want them, fade the edges). I'd check out his tutorial to see how he does it. If you are using GIMP (rather than PS), I've recently put together a tutorial that, among other things, explains cloud patterns I use to create mountains.

Cheers,
-Arsheesh