4 gigs is more or less standard, at least it has been for the last few years. High denominations of RAM though are becoming much more common though. I forget which one it was, but there was a Mac a few years ago that had enough ports to hold 16 gigs of RAM or something ridiculous like that. Really though, if your other hardware isn't powerful enough to back up a large amount of RAM you will bottleneck and your computer will run poorly. It's all about finding the balance.