depends on what you install. Some games have up to 2GB worth of data you need to store. But for me, I have almost 200 apps installed and I still haven't exceeded 3GB used for them. I do have an external SD card, but that's for storing my music collection (I have 10GB on the phone) and movies.
Most apps are only a few megabytes. I have 100 and it takes only a half gigabyte. Games take around 30 mb at most, and some of the 3d fps games go up to a gigabyte or two. Either way it shouldn't be a large issue if you're only holding apps. Music and movies will require more storage, although 16 more would be enough.

I'm running with 1gig internal plus 16 external, and apps are not an issue at all. My large music collection takes up the most space, and and even then my collection pales in comparison to others.