if its usb1 it'll die, (but still get there), if its usb2, it'll just be like running off a 5400rpm drive (or similar) the max mb/s a is usually around 25-30 i tihnk when using a usb2 caddy, wheras your hard drives are more likely to go from 50-80
i run Red Alert 2 : Yuri's Revenge over usb2 (hell, i even run it over a shared network drive at times), because once the map is loaded it uses very little hard drive resources, so it plays fine
something like half life 2 where you keep getting 'loading...' screens would suffer a performance hit in loading screens
my guess is that ingame performance will have very little difference, but loading screens will take longer, and will vary from game to game