Definition

We have chosen this cut-off point to determine what is low end gaming because, as of 2012, Intel's Ivy Bridge (on its impressive HD 4000 chip) and AMD's Fusion chips will provide relatively high quality 3D performance as a standard for virtually all computers in the future. This is in stark contrast to previous generations of integrated GPUs such as HD 3000 or GMA 950, which had very poor 3D performance compared to dedicated GPUs of its time.

What system?

When it comes to Windows system family, Windows 7 or Vista can be used for low-end gaming. The former being more common than the latter, however both are available in 32-bit and 64-bit versions and can be customized to use the classic appearance and disable their composer, Windows Aero, exchanging wasted CPU cycles for minuscule RAM savings.

Another alternative is using Linux system. Desktop environments such as XFCE or LXQt are very lightweight and provide little to no compositions and distributions such as Lubuntu, Xubuntu, PeppermintOS and Bodhi Linux are made specifically for low end computers. Mixed with proper kernel and drivers it may provide a better low end gaming experience than Windows, especially when most open source games have a low system requirement. For playing Windows games on that system, you may also consider using Wine which, depending on the GPU can provide a native support for DirectX 9 games through Gallium Nine under AMD or Intel, as long as the latest version of Mesa is installed (Current version is 17.x.x) and enabling it's support through Staging tab in WineCFG if you are using Wine Staging Nine.

It is also entirely possible to turn your low end PC into a fully fledged console by installing Lakka. It is a Linux distribution based on LibreELEC with the whole purpose of running only RetroArch which is already included and it can be also installed and support Raspberry Pi.