An example is the settings limitation in GTAIV based upon your total graphics memory, regardless of actual card power, or the speed of the memory. My buddies DDR2 4670 1gb allows for higher settings (not performance mind you, heh) then my 4870 with 512mb of DDR5, without resorting to a workaround.
Most games will install, but refuse to run if you don't have a bare minimum requirement. Some of those requirements are based on the actual model of GPU being used, some are based on the graphics specs, like total amount of graphics memory, or DX version. It is different from game to game, how and where these limitations are implemented, but pretty much any modern game will give you crap if you try to run below spec.