BlueSwan
But really, how likely is one to hit that 3,5GB ceiling?
Depends on what you are using. I don't have any huge sample libraries but I will run several several sample based applications in the same project. Kontakt, EZDrummer, Kitcore, Independence, M-TRON pro etc. When I look at the memory utalisation in task manager I am often using all 6GB of my RAM and still need to use a page file.
It would probably work in 32bit but there would be a lot more disk access going on and I would need to freeze synths more often.
Remember you can still get a 64 bit operating system on your new PC and run the 32 bit version of Sonar, so you can get a future proof O/S and run 64 bit Sonar in parallel until you are happy all your mission critical stuff works. That's what I did.