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Well the memory is MUCH faster on the GT and it has more functional pipelines. Although some 6800NU's do softmod to GT's, you can't get over the slower memory that way.
Personally I'd go with the GT. They are getting pretty reasonable for the AGP models. I've seen them as low as around the $340 range.
So with max eye candy in 1280x1024 with or without AA/AF I'd say you might get another year or so out of a 6800GT or Ultra where the 6800NU will start to show it's age.
The 6800GT will still last you quite a bit longer. Once games get complex enough that the 6800GT can't handle them at 1600x1200, you will be able to drop to your 1280x1024 preferred resolution and continue playing.
Not that much longer. Especially if you overclock and unlock the disabled pipes and vertex unit on a 6800nu. You will get performance very close to that of a stock GT.
However, most GT's overclock to speeds of an ultra so you might want to consider that as well.
I own a 6800nu. You really have to consider, how much you play and what you play. My 6800 offers me A lot of Eye Candy and at high resolutions with very nice framerates. "Stress testing" it I was able to run Doom III at 1280x1024 on ULTRA and got around 20 fps. Ran about 55-65 at 1024x768 on HIGH.
You can also you RivaTuner to unlock the hidden pipelines. But you have to realize not every card is able to unlock them all. Some might be broken and sometimes you might get artifacts and such. You can also OC it to GT speeds.
The real advantage I'd say that the GT has over the nu is Better Memory and more memory stock (256 as opposed to 128) 16 Working Pipelines, and OC ability to Ultra.
But like I said before, if you can get all 16 working on a nu and OC it, you pretty much have a GT/Mini-Ultra anyways.
You must also consider price. I bought mine for 295 (MSI) and my package bundle was insane! Its about a +100-150 to go to a GT. And then About +150-200 to Ultra..