so the only difference between all the GT's is cooling? So if i got the cheapest GT, took off its cooling, and added one of them arctic VGA coolers it would be better than the rest?
No not necessarily. You are also paying for quality of the architecture of the board. Certain manufacturers like eVGA will offer far superior quality construction compared to someone like LeadTek or other lesser known brands. Plus some companies like eVGA focus mainly on graphics cards. These companies usually produce higher quality cards because they spend more money and time designing the best architecture, whereas companies like MSI who produce motherboards and graphics cards, although a really good company IMO I would say they are mid range quality. They are really stable boards but generally don't offer much past 'average' as far as options go. I figure it's because they focus on two major industries. Motherboards and graphics cards.
This isn't to say you couldn't OC a no name brand very good because you could just get lucky and happen to get the pick of the litter ya never know. But with a good name brand you're paying for quality too.
EDIT: Oh yeah ak_, also most other smaller companies will put on a smaller heatsink, so in the case between eVGA's 6800GT and most others, then yeah you would definitely have to put on some aftermarket cooling and then you might be able to hit Ultra speeds given you got a good card with decent OC tolerance. Buying the eVGA 6800GT is like getting a free aftermarket heatsink. It's one of those things like a 35w 2400+ XP-M...if you know about it and know how to use it you basically got better than a 3200+ XP same applies with the eVGA 6800GT...some people just assume the Ultra is better and don't read about tweaking the cards out and/or OC'ing