Is there any use for some of these quest items after their respective quests are complete? I mean I hate having to carry these things around in case I need them. They take up valuable inventory space! The items I'm referring to include...

SPOILER-ISH
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Harper Amulet
D'Arnise Keep key
Mae'Var's key
Various Notes/Letters

Also, when canI find my first Bag of Holding?

Just store everything in a chest somewhere (perhaps in your stronghold) - unless you drop thing on the ground they will stay where you put them, so you get retrieve them later if you need them.

Spoiler
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You will find a bag of holding in the asylum.

You won't need the items after you did the quests so throw them away.

Harper Amulet = unless you are continuing Jaheira's NPC quest, I don't think you need this. Or if this is the item you need to enter the Harper Hold, its useless after you've done that quest.
D'Arnise Keep key = unless it is your fighter stronghold, you don't need it. I think it's more symbolic of the keep being passed on to you than any practical use.
Mae'Var's Key = After you open his strongbox or whatever, this is useless
Various letter/notes = almost all of these are for information/storyline purposes only...

And the first (and only, at least in SoA) Bag of Holding is found in Bodhi's gauntlet, not in the asylum.

Bodhi's gauntlet is in the Asylum.

Bodhi's gauntlet is in the Asylum.</font>[/QUOTE]What is Bohdi's gauntlet!? i found my bag of holding...SPOILER
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in a little room in bodhi's maze/prison/thing, it had some golems in it too. i think it was just north of the uber-trap hallway (the one that instant kills you)

Ive never been able to disarm that trap but i ran over it before with the boots of speed and improved haste on my main character. BTW bodhi's maze/prison/thing is called bodhis gauntlet.

You can't disarm those traps (there are 2 next to each other)

White Lancer, the "maze/prison/thing" is Bodhi's Gauntlet. ;)

Man, it was one simple question, now its gone to Bhodi's Gauntlet. Anways, you find the bag of holding there. But just wait till ToB (SPOILER READ AT YOUR OWN COST) You can buy like 3 bag of holdings and a bijillion boots of speed.