Right now the most important thing to upgrade is the graphics card. Eventually you'll want to switch over to something like this: http://pcpartpicker.com/list/38njWX
Get the graphics card first if you can't afford it all at once. Also, if your power supply isn't at least 500W get an EVGA 500B. You will have to upgrade the CPU, RAM, and motherboard all at the same time because your current motherboard and RAM aren't compatible with the CPU on that list, and the motherboard on that list isn't compatible with your RAM.