What should upgrade first?

Ok, i want to upgrade my computer that was built like 2 years ago. I can only run Bf3 on very low graphics on a poor resolution (my screen can go 1920x1080, its 23"). i dont need to run bf3 on ultra for example, but at least with a decent resolution and better graphics.

Im currently running:

AMD Phenom II X2 555 3.2ghz processor
ATI Radeon HD 5700 Series
4Gt RAM (32bit can only handle 3.25gt of it)
Asus M4A77T motherboard
Windows 7 Professional 32 bit

I was thinking what should i upgrade 1st?
I'm low on budget so the price cap is kinda low, so no more than 150euros on one part.