I say go for the 8GB at minimum.
While you might buy all games at retail, demos will usually be the full size of the game - some are as much as 3.xGB in size. There are also many download-only titles (Stardust, Mutant blobs, Zombies vs Plants, Escape Plan, AR Games), DLC sizes, patch sizes (Ridge Racer has a 300MB patch, DLC tracks are about 65MB each). That space will go fast, even if you don't buy games from PSN.
Also keep in mind that these are "hard drive-style numbers" so 8GB on paper doesn't mean 8GB of free space.