You could stuff a lot in a clamshell case, especially if you went with an external optical drive. I wouldn't recommend it though. The cases are notoriously bad. They crack and split very easily. And since you'd have to put new mounting points on the bottom of the case, it would be one hell of a challenge. The cases have very poor heat transfer qualities too. If you really want to do it, just be very careful. I have cracked many cases from just normal use. Here's a couple of pictures for you. The motherboard measures 12"x7" with a 5"x4" cutout for the optical drive. The motherboard is on a sheet of paper for size reference.

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"Far better it is to dare mighty things than to take rank with those poor timid spirits who know neither victory nor defeat." - Theodore Roosevelt

-USMC Corporal Hess Sherrod B:6/6/84 D:4/6/09 I miss you, my brother.