What should i upgrade first?

Hello
I'm not very good with hardware so i'm not sure what should i upgrade next. Can you guys help me? I built my computer 3 years ago and is still decent but it needs an upgrade (at least for dolphin)

Specs:

CPU: i3 540 @3.07Ghz
Video card: ATI Radeon HD 5570
MoBo: Intel DH55HC
Ram: 8GB (Enough)

I can run pretty much all games with decent graphics and smooth gameplay and most of the games work on dolphin (until i use LLE or play any metroid games)

Thanks!