It will always depend on what you are running in 6.5.
I have 32GB of RAM and have yet to run out because of Reason, but it will probably happen eventually if I don't start converting more midi tracks to audio.
But why get a Mac when you will get more RAM and a faster CPU for the same amount of money if you buy or build a windows machine? Better yet, build a Windows machine and make it compatible with the Mac operating system like I did (I did it mainly because my audio interface only works for Macs and I didn't wanna buy another). Then, you will get the best of both worlds.