Alternatively, you might choose the Gustav Holst-inspired film soundtrack classical end of the spectrum, also encompassing whatever type of music Enya is:
Neither of these has ever really sounded like the music that is going on inside my head when I read fantasy and SF fiction or play D&D, though. Fantasy/SF-inspired metal is generally too self-consciously doom-laden, and the classical-inspired stuff tends to be too faux-portentous or grandiose (much as I love me some Wrath of Khan OST).
These things are all a matter of taste, of course, but I more and more am coming around to 1970s jazz fusion as my personal backing music for D&D - specifically, the music generated by the weird conglomeration of virtuosos surrounding keyboard legend and crazed scientologist Chick Corea, particularly the outfit Return to Forever. What is it about this music that screams JACK VANCE MOTHERFUCKER so loudly in my ear? Is it just that I can totally imagine a Dying-Earth spellcaster mixing potions in her laboratory to this soundtrack in some long-forgotten late-70s exploitation flick?:
Or this kind of sounds like the background music playing while somebody is travelling across the countryside in a never-made anime version of Lyonesse?
Or that this actually does sound like an alien chase in an Arabian desert (wait until the 2:25 mark)?
Or that it sort of feels like Cugel could be in the audience here, nodding his approval?