With a budget of $600, this is what I came up with to upgrade: http://pcpartpicker.com/p/1G9ZC
All Black Scheme, CPU, MOBO, GPU, Case, and PSU are necessary. I already have a good enough HDD along with some decent RAM (6GB not too big, not too little).
It's be great if you could fit a NIC or some fans in there. Thanks.
EDIT: Need a big/bulky case that won't fall over when someone accidentally touches it. Will also be overclocking GPU.