I ALWAYS equip my Mages with ranged weapons, whether they're proficient or not. Mages have such crappy THAC0 that they're hardly likely to hit the enemy anyway, so I like to give them a Sling just to make sure they're not going to try to enter a melee. A rough-and-tumble brawl is the very worst place for a Mage to be, and all offensive spells where you have to be right next to the guy (Burning Hands, Ghoul Touch, etc.) are worthless. Of course, now that I've said that, Xan with the Moonblade and the right Robe isn't that bad--I've had him going toe-to-toe with Gnolls and coming out only a little the worse for wear--but he's an exception.
I prefer Slings to Darts & Throwing Daggers, because for some reason you can't put extra Darts into the ammo slots, so if your Mage is chucking 3 Darts a round, before you know it he's run out of ammo and is going to run up and try to punch out an Ogre. Bad move. Even in BG2, you still can't put Throwing Daggers in the ammo slots. (But on the bright side, there's at least 1 Throwing Dagger that returns to your hand after you throw it.)
Pschub, don't worry, BG is the kind of game where you start it once, play it for about 5 hours, realize what you SHOULD have done, start over, play for 9 hourse, realize what you should have done THAT time, start over, play for 15 hours, etc. Don't worry, you'll get the hang of it. Your strategy of using 1 guy to run up & attack the enemies hand-to-hand (the type of character we call a "Tank"), while the other members of the party fire ranged weapons, is very sound. Just play with the different party formations (Don't make your Mage walk in front), get the hang of all the spells, and hit Pause more often during a fight and you'll be fine. And just because the plot tends to lead you toward taking certain people into your party (like Khalid and Jaheira), that doesn't mean you have to keep them.
And Lemernis is definitely right: You DO need a Thief. Emphasis on NEED. If you think you're having a tough time now, wait until you run into some nasty traps that will just fry your sorry ass, or until you find a chest that
might contain some wonderful magic items--but you'll never know because you're not strong enough to bash the lock open.
Lemernis: Your long post detailing the NPCs and the spells is spot-on, although PLEASE be more careful with spoilers, especially big fat ones like Algernon's Cloak and map coordinates. The game is MUCH better when it's allowed to retain a little more of its mystique; I once accidentally read the exact location of the Helm of Balduran, and my sense of accomplishment from obtaining such a great item was completely ruined. Thanks for the praise on my accuracy, though.