40GB is enough. Actually, I think its still too much seeing as Sony arent offering downloads that are likely to take up that much space. It really just depends on how much stuff you want to put on there.

Like it matters anyway, if you buy a 40GB and find out you would of prefered an 80GB just remove the hardrive and put in another. I havent done this but I know people with PS3's rocking 120GB just from putting in spare hardrives they own.