Photoshop editing HUGE files. best PC spec? 32GB or do I try 64Gb ram?

Whenever you make a change to an image Photoshop saves the previous version so that you can "undo" the change.   If you're editing 3GB images then there's only so many changes that the system can buffer in memory, so they end out being written to your disk.   It doesn't matter how fast your computer or disks are, saving a 3GB image to disk is going to be a slow process.

For images of that size you might think about turning off or highly curtailing the number of history states that Photoshop saves (use Edit -> Preferences -> Performance and change the values in the "History & Cache" section).  You'll loose a little time every so often when you goof badly enough that you have to go back and reload your original file and redo your edits up to that point, but the savings in edit speed may well be worth it