Help to upgrade components: Gaming Needs.

Hello All,

So I built this comp back in 2008 and have changed maybe the RAM sticks and the video card once (one old graphics card for another). However, I'm now ready to invest a little bit more than 75$ in changing over the main components.

I would like to change over the processor, motherboard and graphics card... probably the RAM later on, but those main three.

Here's what I'm currently working with:

Processor: 2.40 gigahertz AMD Athlon 64 X2 Dual Core

Motherboard: ASUSTeK Computer INC. M2R32-MVP

Graphics Card: ATI Radeon HD 4770

I would like a quad-core CPU if it's cost-efficient, and a reasonable graphics update so I can max out as much as I can on todays games. I do a lot of graphics school work also, OpenGL and such. Are there any reasonable suggestions/upgrades that keep my total cost under 600-700?