No matter what you get, it'll run at 200MHz normally. The 'runs at 266MHz' tag basically means it runs stable when overclocked to that speed: tougher programs run better with higher overclocks.
If you want to play Quake, Descent or complex NeoGeo, SNES or GBA roms, I recommend getting the 266 version. Otherwise, just get a random one - the vast majority of gp2xen can OC to 240MHz, which is the most you should need for almost every program.