If you're wanting to do gaming most pre-builts have low quality parts and 9 times out of 10 they boast having a good gpu in one, but having a shit cpu, and vice versa. That's why i recommend building your own machine as its not very hard, and you have tons of flexibility to fit your exact needs.
Yeah, Gaming on a prebuilt on isn't worth it. You can always hire someone to build it though I've built a few computers for people for around 70 dollars.
Customize the parts you want. This is the computer that they used to use in MLG tournaments. Not the best parts, but definitely better than what you'd get at best buy or walmart.