GET THE AMP OFF THE FLOOR
that's first and foremost. put it on a bar stool or a chair or something. but get it off the floor.
i'd put one mic (an sm57, senn e609, md421, m201, whatever) up on the cone of the amp and position til it sounds good.
for the room mic, play some and move around while you play (or while the guitarist plays). find the spot where the sound from the amp sounds best--could be up high, down low, whatever. put the room mic RIGHT THERE.
make sure you account for phase relationships between the mics, and you should be on your way.
but unless the amp sounds good in the room in the first place, you're fighting a losing battle. you can't record what's not already there.
cheers,
wade