cpu/mobo and gpu will be your best bet. Should be pretty easy to get a decent cpu/mobo and a mid tier gfx card for about $500 to $600 depending on sales. I believe new processors, mobos, gpus and memory are going to be released this year. Intel is supposed to release the haswell-e chip and with that DDR4 memory is also supposed to be released. Then Nvidia is supposed to release the gtx 800 series. It would be in your best interest to wait a bit for those before upgrading.