Negatives

well, you are trying to work on strength to be able to do the pull-ups. as far as your workout with free weights goes, you'll need to decrease those reps and increase that weight. try sets of 8-10, proegressing eventually to 6-8 and so on. also, the muscles involved in the pull-up are the lats and biceps, also rhomboids, traps, etc. you still should do triceps as part of your workout, but they are not specific to the pull-up. keep doing the db curls as indicated. you could do some rowing exercises for your back such as:

bent-over db row
bent-over pronated db row
as well as exercises such as:
reverse flyes (some call them bent-over lateral raises)
lower trap exercise (lying on your stomach, hold the db's with your palms facing eachother, lift arms up in a v fashion (overhead)....(hope this is clear)...:(

another thing i found worked for me was doing negatives, ie, the down part of the pull-up. put a bench under your pull-up bar and jump up to get into position, then slowly lower yourself down...jump up again and slowly lower yourself...do 8-10 reps like this. this will get the specific muscles involved working eccentrically (you are stringer eccentrically than concentrically so you will be able to do this ;) )

good luck!