5670 doesnt come in low profile, The 5570 is slower than a 9600GT.
Sparkle makes a low profile 9800GT. No other brands make one though (that i know of).. so im worried it might run a little hot on a low-profile card.
Speed in COD: (this is taken from the 5570 article on the home page)
What kind of power supply do you have? or if its a prebuilt--what brand/model is it? (i.e. dell PSUs are usually better than HP)
If your PSU is on the low side I would go with the 5570 to be safe, if it seems decent the 9600/9800GT would be faster for gaming.