Standard edition can support up to 64 GB of memory: http://msdn.microsoft.com/en-us/library/ms143685.aspx

As far as deciding CPU vs Memory it depends on what your systems are doing. Are your processes CPU bound (i.e. lots of aggregations)? If not, with SQL Server more memory is pretty much always better. Some things to watch out for when virtualizing is that when you setup the VM with X amount of memory, talk with your VM admin and make sure you're ACTUALLY getting that much. VM admins can restrict how much memory your system actually uses as opposed to how much it "sees", same thing goes with processing power.

Also why do you need to run SQL 2000 and SQL 2005? Can your 2000 databases just live on 2005 instance and the databases stay running in 2000 compatibility mode? If so, try that out. Also if you're separating versions for the sake of breaking them apart, look into creating named instances on the highest version level you can and have your applications point there instead. Hopefully this makes sense and helps you out.