Memory Allocation in Shared DB & IIS Environment
When there's nothing else on the machine besides SQL you need to leave at least 2-4 Gigs of RAM for the OS.  When running a bunch of different IIS sites you'll need at least 4 Gigs for the OS and IIS.  As some of your machines have only 4 Gigs that's a bit of a problem.  Can you add some RAM to these servers to get them up to 16 or 24 Gigs?

I'm assuming x64 here?