Maximum PC

Which of these nVidia GeGorce 6800 appears better. I'm looking for one with a good and quiet heatsink.

http://www.newegg.com/app/ViewProductDe ... 202&depa=0

http://www.newegg.com/app/ViewProductDe ... 193&depa=0

If I was you I would choose the Ledtek one as Beomagi suggested.
Just for the fact that they have a reputation for successful unlocking.

What I like about the is the big heatsink which also covers the back of the card. Also, some people have complained that the fan on the EVGA model is really loud. So I will probably go with the Leadtek.

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. If I was going to get a 6800 I would save up the extra $100 bucks or so to get the GT Version with 256 MB of DDR3 mem. But I am no expert any one else feel free to disagree.
You can also do a search and see what opinions have been voiced in the past.
But if you are set on one of those two I think the Leadtek with the bigger heatsink would be cooler and quiter and it has the best bundle.

I would unlock the pipelines and overclock it so that the performance will be closer to a 6800GT than a 6800nu.

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.

The speed doesnt alwaays determine hoe fast a card performs. For example. Remember Geforce 5900s? They have screeming high Mhz for their cores and memory but they still got their asses kicked by 9800 cards which were clocked much lower, about 150-200 Mhz slower.