It's not quite as simple as that. It can depend on what codec you're editing, what resolution and so on.
I've been doing a lot of h.264 stuff lately, so the processor is the main bottleneck, so upgrading the processor is the right move for me.
Assuming you're also mainly using h.264, for you, it's probably going to be:
CPU (or going to a multi cpu computer)
RAM
SSD HDD
probably in that order.