Memory is the number one improver of performance for Photoshop and large files. Max out RAM first. An SSD is helpful compared to a spinning platter, but (again) only if you're swapping to disk. Photoshop is a little bit of a pathological case because it manages its own memory file (meaning that the OS doesn't have to seek back to the directory areas when writing to the disk as much). I haven't noticed too much of an improvement for an SSD over a spinning platter with Photoshop because I don't normally have to swap to disk. I tend to work on fairly small files that don't make the system swap a lot, though.