I would choose neither I would go with the 6600GT
both the chipset core Speed (325mhz for the 6800 vs 500mhz for the 6600GT) And memory Speed (700mhz for the 6800 vs 1000 mhz for the 6600GT) Plus the 6600GT is Cheaper.
chipset is faster - but you're forgetting the number of pipes. Did you know that the 9500 was a MUCH slower core than the 9600? but the 8 pipe core (essentially the 9700!) pushed it slightly above the 9600(4 pipes).
the memory interface of the 6600 is 128 bit - vs the 256bit interface of the 6800.
still the 6600gt is a fine card. if you can get it for under $200, it's a tossup. The 6800 unlocked would net you a resolution higher, maybe a res with some aa. Either way you're pleased as punch.