More memory will probably only help if it is a 64bit OS. Much of the overhead of the multiple instances is going to be in memory that SQL Server cannot use AWE for.

Adding memory and enabling AWE is going to take more physical memory away from the SQL Servers to manage the AWE memory map. This is one of those situations in which more memory may actually hurt.

If you intend to add memory, Steve's suggestion about consolidating the instances is something that you may need to do to get any benefit.