Funny this topic came up. I just got a bunch of detectors from Home Depot last time I was there.

They have these really nice ones (which I didn't have the money for) that are inter-connected "wireless" so if alarm A goes off in your basement it sets off all the other alarms to alert you sooner VERY COOL idea but for $30/each a little more than my budget allowed.

In terms of carbon monoxide and smoke detectors I`m a firm believer of one in every room. And your hallway and shared room/great room. (I don't have any plans to put any in the bathrooms right now).

I also suggest battery powered