what is my pc capable of doing?
With the nature of having Asperger's Syndrome it would be hard to represent the exact quantification of what your computer is capable of doing. Although you have a dedicated GPU(Your GeForce GTX 210) it is severely outdated;I'd highly recommend you update your GPU to something like GTX 770 or 760Ti or even a mid-level AMD 7000series GPU. With the current GPU I'd suspect that it would be outperformed by many modern day in-CPU graphics processing units. Along with the graphics card I'd recommend you upgrade you RAM to 8gb as it is extremely affordable now a days. Because of the nature of the modern parts they require a higher power input to operate optimally. I was not able to find the exact wattage and voltage the stock Optiplex 990 comes with so I recommend you also upgrade it so something around a 750W PSU. If you are also planning on storing important documents I'd also recommend you purchase a new hard drive as the mileage on the current one is most likely unknown so the current hard drive could stop at any moment. Because this whole uplift would be a pricey investment I'd recommend that you have someone with computer experience put it all together. Another option would be to purchase something like along the lines of a Lenovo Y40-80(high performance laptop). I hope I helped you out with your question if you would like links to recommended products, as the majority are manufactured by different companies, feel free to ask.