As for the actual topic lol, no it's not a stupid question. Sony is making so many PS3 versions that I can understand it being confusing on which does what as each seems to have a capability or lack of one that another has.
The 60 GB has BC, and the emotion engine. I could be wrong but I believe the 80 GB is identical but has a slightly different CPU. Both are backwards compatible, and neither suffer in graphics or processing power.
The 40GB was introduced in place of the 20GB that wasn't selling as the new cheap version. Since at that point only 20GB seperated the new 40 and 60, and it would still have been $100 difference they upped the 60GB to 80GB as the new higher priced model, and then removed backwards compatibility with PS2 from the 40GB model.
So pretty much the 60GB = 80GB, no real difference. And the 20GB = 40GB. Pretty much the same machines at the same price point just with 20GB difference on both.