Unless you are simultaneously streaming multiple HD-quality movies all over the place (to AppleTV, to another computer, from/to a media server) I don't think you need to upgrade a switch at this point.
The quick bursts of activity are wasted on Gigabit. That's more for sustained gigabytes of transfers happening rather frequently, or for lots and lots of smaller groups of networks that might need to share space with each other while slinging a bunch of data all over the place.
Your setup would have a hard time making gigabit even notice it was being used.
Think of the slowest link in the chain, typ. the cable modem, and that is the limiting factor of the whole network - again unless you are slinging a LOT of data around inside to your various computers its just a waste.