the only thing holding me back from buying a 6800NU..

it only has 128mb memory. is this a big issue? or should i wait and get a 256mb card?

I doubt you'd find much difference even if it did have 256 megs.
I say, just get the one you want.
don't worry if it has 256 or not.

Why get the NU? I would get the GT

I was thinking about getting GT too at first but my with my current processor it will just create a bottleneck to the card. I think NU is not a bad way to go, someone just post on unlocking pipelines to 16 and for that price i think it's not a bad choice.