I'm not the best in this, but if you need some help just ask
If you really want to do graphics intensive stuff, the minimun should be a quad-core CPU, 4GB of RAM (8 if you can) and a GPU with at least Pixel-Vertex Shader 4.0 and 1GB of VRAM (>150$ GPUs). I did a quick search on amazon and I can see good computers at less than 700$. You just need to do some research on different sites and maybe ask a few experts.
I can't buy a new computer because my parents won't let me buy it, but I did so many searches on internet for that stuff, and finally found a rig that would be great for me.. If you're not sure about building it by yourself, as you can see there are a lot of great pre-assembled pcs. The problem in pre-assembled ones, is that's there always a bottleneck, usually it's the GPU which is the most important part nowadays unless you don't play games or do graphics of course..