Building a PC is unnecessary inconvenience, and if you've never done it before, it's easy to do something stupid like buy an underpowered PSU or the wrong type of RAM or something. In a prebuilt, RAM is a pretty good proxy for whether or not it's going to be a decent gaming machine. If it has 8 GB of RAM in it, it's a budget model with an absolute joke of a graphics card in it. But it's not 1999 any more, where hardware becomes obsolete every 10 minutes, and buying the wrong model of Geforce means that in 18 months, you can't run any new games. To be quite honest, if you spend about $1000 or more on a new PC with a discrete graphics card, you'll be fine.