I am not expert but I have been asking similar questions for a long time, and I know the kind of answers you are going to get. First off, you might be able to get a prebuilt PC that is good for single monitor gaming. It won't be anything amazing but it won't be awful either. Secondly, a $500 custom PC might get you the performance of an $800 prebuilt. Prebuilts are overpriced because of labor and the parts they use sometimes aren't the best. Not all the time though. Not only are prebuilts generally overpriced, they tend to have issues. If you build it yourself, you will know it like the back of your hand. When it is time to upgrade, you won't have to worry about taking it apart the wrong way because you put it together. If you don't know how to build a PC yourself, I am sure there are step by step guides all over the internet and maybe even on this site that could help you. So you could spend $500 on parts to build it yourself, and get so much more power out of it.