It'd be very hard to hit the limit even with a 40GB. The HDD space is used mainly for other things... downloading game trials, movie trailers, storing pictures, movies, music, etc., caching streamed video or music from your home network. The games themselves don't take up a tremendous amount of disk space.

Personally, I still think you'd be fine with the 40GB, though I imagine Sony plans on making heavier use of the HDD in the future as new features and functionality are released.

Epic 80-800: HG Cherry