Both are a huge waste of money unless you have a pressing desire to play DX10 games at unplayeable framerates

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Get a 7900GS or X1950 Pro instead as they're about the same price anyways and a crap load faster.
If you need something thats low power and cool that requires no external power then get the 8600GT, otherwise spend your money on a much faster DX9 card cause you ain't gonna be DX10 gaming on either of those cards.