My I suggest. (Never done this before, contracts always ran out.)
#1. Do it like the guys said. In person.
#2. Volunteer the info about past projects in person, don't put it in the letter. (8 months after the end of a contract I was still getting calls about how to do this that and the other. They even asked me to go in and service their computers a few times too.)
#3. The guys are right about the gratitude. Say you have learned alot, say you are sorry you have to leave. But keep it short. The letter should be something they file away. Just for a record.
#4. If you decided to re-structure it, and re-type it, post it back on here and tell us to beat the $h1t out of your grammar spelling. There's no harm in getting it completely right. (Hey, there's most likely a couple of english professors on here or something. You can probably get it checked in both English +"U's" and English-"U's")
Mobil doesn't look like the correct spelling, but what the heck would I know.. I'd just use "Cell"
Um.. It's not bad, but the second sentance being a run on isn't perfect.
A good letter of resignation, under good leaving terms will no doubt secure you a good letter of reference, if not a Job incase of any problems with a new one.
No matter how much they are pissed cause you left, if it was under good terms and they are a smart bunch they will relaise they have 4 years of training and processing spent on you and most likely hire you back.