If I just hear a cd in a friends car, Ill go dl some mp3s, listen to em, and then probably (more often than not) go buy the cd. Either way, I delete the MP3s (either to get rid of the crappy artist that sucks a monkeys dongle, or to replace them with higher quality MP3s direct from my cd).
If its a band that I love and already have several cds of (I have very few single albums from an artist. Im something of a completist, I like having all of an artists killer albums ) then I buy it outright the day it comes out. (bands like Remy Zero, The Ataris, New Found Glory, Jets To Brazil, Radiohead[though ive later resold the latest two radiohead albums..blech], Braid (now Hey Mercedes), Get Up Kids, etc......come on, you knew I had to put that in )