These guys are giving good advice, so I'll let most of that stand.
The highest use items in our shop are the blast cabinet, followed by the die grinder. If you will do much of either (and if you are doing much of any restoration a blast cabinet is worth its weight in gold), you will want more CFMs for sure.
But Marco hit the most important nail on the head: look for quiet. Even in our big shop, the compressor is incredibly loud when it runs. Next time I build a shop, I'm going to build a little outdoor room to house the compressor in, just to keep it quieter in the shop. It's really hard to work sometimes with anyone else if the compressor is running.