Adding RAM is completely unrelated to the /3gb boot switch on 32-bit Windows. Many people incorrectly assume that the /3gb boot switch lets Windows use the installed RAM more efficiently.
If you’ve never had Pro/E crash because it tried to access more than 2GB memory, then you don’t need the /3gb boot switch. This switch controls how much virtual address space each process can address.
Virtual address space and RAM are not the same. A process’ virtual address space exists both in RAM and on the hard disk (page file). The Windows memory manager handles swapping memory pages to / from the disk so that any memory pages required by the CPU are loaded into RAM.
So you can see that using or not using the /3GB switch has no effect on how Windows uses the installed RAM.