There are more 5600's than that.
5600 128MB - the slowest of them all
5600U 128MB - revision 1, faster clock speed than the 5600 non-ultra, but slower clock speed than the revision 2 Ultra.
5600U 128MB - revision 2, faster than revision 1 due to higher core clock speeds, and also overclocks better
5600U 256MB - not sure about core speeds, but the extra RAM does not produce any improvement in performance at the current time, and by the time it does, it may not be fast enough to run whatever game.
The 5600Ultra 128MB card with speeds of 400MHz core and 400MHz RAM (800MHz DDR) is the best 5600Ultra to buy as it's the fastest, and will overclock the best.