I believe I'm writing in regard to how to write an informal business letter.

Here is my situation:

I recently contacted an advertising agency through their "general information" email address to request their company brochure. To my surprise, the creative director and senior vice president of the company replied back. As an aside, I informed the creative director of my motive in requesting the company brochure: I would be looking to leave my current position in the next year to move to an advertising agency, and I thought the particular agency at which he was working held a lot of promise. The creative director suggested that I send him some samples of my work when I got the chance.

And that's where I stand now. The important thing to note is that during our email correspondence with each other, the creative director and I were very informal—we spoke more like friends than like future boss and employee, so to speak.

So I am ready to mail my work samples, and I'd like to include a letter detailing some of the work I will have enclosed. How should this letter be written? I've used the creative director's name in our emails with each other, saying "Hi Rick" and using other similar gestures. I'm just wondering if this letter should be less formal than a standard business letter, and if so, how I should go about doing that. Any advice, or links offering advice, would be greatly appreciated.

Thank you for your help.