Also think about your relationship with your manager. Would he or she have reason to think that you'll waste your last two weeks or distract your co_workers instead of doing what you can to make the transition seamless? If so, you may be asked to cut your two weeks short.
Why might you still want to write a resignation letter? Many experts believe that writing a formal or even a basic resignation letter when you're leaving your current job helps make you seem more courteous and professional. However, the real reason to write a resignation letter is to create a paper trail documenting that you have given notice (if your employer requires two weeks notice or some other amount of notice) on a certain date, and to formally kick of the process should there be any legal hiccups (quite rare) with incorrectly processing your final paycheck and last day of benefits.