In a room the size you're dealing with, even a single PCi would be great all the way up until you hit loud levels, but if you really wanted to shake your room apart, a single PC-Plus would do it in that size of a room.
Two subs can help with room modes, but sometimes can even make them worse depending on placement and the acoustic properties of the room. If you go with two I would definitely look into getting an EQ with it as that will increase the subs efficiency greatly. Something like the audyssey can even automatically equalize for multiple listening positions ensuring you get the best sound possible.
If you're a big bass fan and want the best performance, I would recommend a PC-Plus and get an EQ. I'd only recommend 2 x PCi if you're trying to wake up the neighbors across the street, which sometimes is the goal ;^)