I've run Fusion with 2GB RAM on my Macbook. I found that one virtual machine was usually fine, especially if it was 512MB RAM or less. However two would cause some slowdown, even if they each only used 128MB RAM.
The 2.0.1 update made this a bit better, but I still saw some issues with multiple VMs running at the same time.
But just one was totally fine. And if you are doing most of your work in the Mac and running things like Outlook or other specialty software you might not need your Windows VM to have as much RAM as you originally though.