performance is pretty much the same across all the 6600 models.
difference is usually the board design itself and the cooling solution. you have to look at reviews of each model to see what is under the hood.
some run cooler or quieter or have a factory overclock or other design changes.
there is a "reference" design that amd/nvidia send to the brands. they usually use it and put multiple cooling solutions on it. then they make a custom board and multiple coolers for it. that's why one brand can have 6 or more models for the same chip.
individual differences are shown through reviews as they compare them to other models and chips.
but again, for the 6600 cards, they all perform about the same. grab the best price or the one...