You could always leave the 150's where they are at as rear surrounds and buy a set of Inifnity Outrigger Jr's to put as side surrounds. They are intended as outdoor speakers, but make a very usable ceiling or wall-mounted surround. A second option would be the Infinity OWS-series on-walls you could plant pretty much anywhere for mid-surrounds. My local Circuit City has them for $ 115 each a couple of days ago.
Bipole and dipole speakers can get to be a little TOO much science, since one works great for HT but lousy for music while the other is wonderful for music but sucks for HT. Even the switchable ones can be a pain, since the receiver won't switch them automatically. If you are going to be dedicated one way or the other, then by all means jump right in, but if you need some flexibility, standard speakers are the way to go.