What part of my pc should I upgrade first?

[Jcdoe]

I'm running the following hardware:

Core 2 duo e6540 @ 2.4 ghz
Geforce gts 250
Nforce 780i mobo

Also, would I possibly benefit from overclocking? I know dolphin only uses 2 cores, so a quad core setup would be overkill. But then, I wonder if a new CPU/ mobo is needed to deal with bottlenecks.

Advice please!