The 6600GT's memory clock is also higher than the 6800LE. Performance is better on the 6600.
As for 6600GT vs. 6800 non-ultra, they tend to be similar until you start loading the cards down by turning on AA/AF and going to high resolutions; at that point the GeForce 6800 non-ultra leads the two cards in performance.
bhair's post is entirely correct. In addition, not every Geforce 6800 non-ultra will do the softmod well. I can unlock the additional four pipelines on mine, but I get graphics corruption during 3DMark05. It's a gamble, not a freebie. Even so, the GeForce 6800 non-ultra still has twelve pipelines as opposed to the eight of the 6800LE and 6600GT.
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