Great tutorial on recording heavy guitars
Start there.
Then, a few general pointers/comments.
-your mic is probably fine. A dynamic mic up close is the most time-honored way to get a good tone on tape. That said, if you want to throw money at the problem, grab a SM57, the industry standard cab mic.
-remember that you "listen" to your amp from about six feet up and maybe 10 feet back, in the room, while your mic is "listening" from an inch or two away from the speaker. In other words, what your mic is hearing is VERY different from what your ears are; you may want to consider EQ'ing your amp to sound good from right out front, and not from the room.
-always double track. At least. Two takes of rhythm guitar panned left and right will sound much bigger than one.
-likewise, cutting the gain a bit will make your doubled rhythm parts sound, unintuitively, even bigger.