only a handful of games install 2GB or more; some installing none while others install a few hundred MB. afaik, there are only 2 games that install more than 2GB, and those are UT3 and DMC4. so, i figure an average would be around 2GB or less. now, if you plan on having 10-15 games, thats 20-30GB. i figure a 40GB PS3 will actually have about 32-33GB after format ("40GB" is actually 40,000,000,000 bytes, which equals ~37GB - then the PS3 reserves some space for cache and the OS etc.). with that said, even if you have a fairly large library of 15-20 games, theres a good chance you will not be playing the games you purchased earlier on, so you can always delete the game files (which are separate from the saved files - you'll just have to install the game again when you want to play it). now factor in how many demos or games from PSN (which are usually around 900MB, more or less). if you don't plan on having that many games, then i think 40GB will suffice.
and like others have said, you can always upgrade your HDD, which is quite simple to do.