All-in-ones are not recommended for gaming. As said above, most of them are just immobile laptops. They suffer the same drawbacks that laptop gaming offers such as cooling issues, lack of upgradeability, and high cost to get a decent GPU, all without the advantage of mobility.
I would say just make some room on your desk or somewhere nearby for an actual desktop. If you are absolutely set on an all-in-one and are willing to pay a huge price premium for one that is specced for gaming, look for something that has a top end laptop GPU like the Radeon HD 7970m or GTX 680m. That will get you something equivalent to a midrange gaming desktop built today. Just be aware that you probably will not be able to upgrade the graphics, so you will have to buy a new computer in a couple of years when the GPU starts to fall behind the latest titles. You may be able to get more life out of it if you are willing to accept reduced detail settings as time goes on. Ultimately, a desktop is much better for gaming, you can get better cooling solutions, so you don't have to worry about overheating issues, and you can drop in a new graphics card in a couple of years when you feel you need more GPU horsepower.