Well, I'm watching a sub right now. 8", dual 4 ohm voice coil, and 4" of depth and it's ideal sealed enclosure is 0.35 cubic feet! If I connected this up to my amp, that would be 250 watts RMS going to a little 8" sub. This means LOADS of headroom and plenty of power to spare.
I will be replacing the front speakers as well and potentially the rear speakers too. The front speakers, I want to use Infinity as I love the way they sound (very soft and warm tweeters due to a textile dome tweeter). For the rear speakers, I found a pair of 6 1/2" 4 ohm woofers that might just work perfectly for adding a bit more midbass to the system, yet not moving the sound stage off the dash. We'll see.
I go in on Tuesday to have my foot X-rayed again. If the bones have healed pretty good then the doc will put me in a walking boot, so I'll be able to do a LOT more outside. If the bones have not healed enough, then the doc will put me back into a cast again and I will have to wait 2 more weeks.