I think a logical procession from mr. Beer would be an extract kit with specialtry grains. Those kits all make 5 gallon batches. They have some "Brewer's Best" kits which are pretty good (if you get them fresh), or you can try some of my favorite kits from austinhomebrew.com. Here:
http://www.austinhomebrew.com/index.php?cPath=178_452_43&osCsid=ba0cee1b8aec8d502803526329e01ba6 I've done the Fat Tire, the Pete's Wicked, and a few other "clone" kits. Make sure you get the "extract" kits, and they have steeping grains with them along with great instructions.
I like that they have hundreds and hundreds of choices, so you can make what you like. A couple of suggestions- stay away from lagers for the near future, and make ales; and get the dry yeast option until you're comfortable with yeast starters and handling liquid yeast.