minor spoiler on crossbow location . . .
heavy crossbows have a higher speed factor so shoot later but have a damage bonus compared to light crossbows
best crossbow as far as i'm concerned is the 'light crossbow of speed' available to buy from the smith in beregost
full list of bg1 ranged weapons here
http://mikesrpgcenter.com/bgate/ranged.html
EDIT: ooh - new piccie . . .