since the 6800 GT and Ultra are the same card, overclocking the gt...

They are NOT the same card.

And where did you read that PC 68GT has fewer pipelines? WRONGO !! It had 16, just like Ultra.

It was the lower versions that had 12 or 8.

Highest spec components went into Ultras, Cores with best ability as well as RAM chips.

The GPU voltage is same on both Mac cards, but I would be very careful overclocking such an expensive card.

Nvidia 6xxx series cards have proven rather unforgiving. And the one BIG difference is cooling.

Please show me a link to this info.

Here's one showing you to be in error:

http://www.digital-daily.com/video/geforce-6800gt-roundup/

or this one:

http://www.pcstats.com/articleview.cfm?articleID=1656

"The 'NV40' GPU is the backbone of the 6800-series, and comes in a couple flavours; 6800 (12 pipe lines), 6800GT (16 rendering pipelines, 350MHz core), 6800 Ultra (16 rendering pipelines, 400MHz core),"

The 6800GT had 16 pipes, PC or Mac. You are simply misinformed.

And when something can be numbered it is "fewer", not "less".

You might try researching things before typing.

As David has pointed out, the GeForce 6800 GT has the same number (16) of pixel pipelines as the GeForce 6800 Ultra. The Ultra is clocked and guaranteed at higher speeds due to higher quality (and more expensive!) cores and RAM. You may be mistaking the AGP GT for the AGP GS, a card that is masked to 12 pixel pipelines.

Certain cards like the ones from BFG may come with coolers beefier than the reference design.

Overclocking is an easy way to void your warranty; only some manufacturers guarantee their cards for higher speeds. If you do so, be careful. If you damage your card by aggressive clocking, you may find yourself out of luck.