I usually find my apps are CPU bound rather than memory intensive, so it pays to test this out. Easy way to find out is to run apache bench locally and watch the server performance (Use perfmon on windows, not sure what the linux/unix alternative is). Its a crude performance metric but will point you in the right direction as to what you want to upgrade first, you can validate your findings with a more complex tool like Webload/JMeter/Wcat if you want to make sure.
