Not much.
Sure, you can buy a wallet with an Atari 2600 image imprinted on it, but if you really want to show your nerd cred, you'll make your own wallet out of an actual 2600 game cartridge.
It's actually pretty easy. Want to know how? Watch Bob Clagett's video (or just read the blog post on his website) and you'll have your own cartridge wallet in no time.
Now I just have to go dig up an E.T. cartridge from the landfill.