I voted for 1 GB as well. Most of our clients that run a handful of sites use 512MB of RAM with cPanel and that seems to do the trick while customers with 50 or more sites tend to go for 1 GB or more of RAM.

This also depends on the traffic trends you are seeing, if during your peak times, the sites are experiencing quite a slow down then I would suggest going with more, but if it seems smooth then let it be. This is the great thing about virtualization, you can scale your resources when you need it.