What is a simple light weight OS I could put on my other computer to play roms. What I want is an emulator game console and I wanna have my computer hooked up to my TV and a usb controller and be able to select and play games by using just the controller.

Just get a gba and a gamecube and gameboy advance player for gamecube

Emulator games even with perfect computer specs NEVER feel the same as the real game. Just put money into the real experience.
If you want mario, get a nes/snes etc

If you want old pokemon, get an old gameboy or gameboy advance/etc

Bumping for interest. In an emergency you could just use a regular emulator and a HDMI cable to hook the PC to the TV.

Got a Wii? Great, now you have an emulator for everything GBA and under

get a raspberry pi. i use it to play games like pokemon and stuff