Remember the 80/20 rule. Most of the sites only have a couple of clicks per day. Right now we have around 15 sites and two rather heavy apps on a 512 slice. We always intended to upgrade it, but it turned out not to be the case. On a 256 slice we had all the sites confortably.

As for the few sites which get a lot of traffic... they should also generate a bit more income and an upgraded slice.