For $300 you can get a 7800 GT that will outperform the 6800 GT 512mb Edition. Becides, I thought they only made the 6800 Ultra in 512mb...and the Ultra line got discontinued by nVIDIA.
7800 GT - 20 pixel pipelines and 8 vertex shaders.
7800 GTX - 24 pixel pipelines and 8 vertex shaders.
Don't even think about unlocking the 7800 GT's pipelines. They're completely locked down and it's impossible to do it. nVIDIA actually learned from their mistake too. When the 6800 Ultra and GT came out, the GT was bascially a underclocked Ultra, but it had the same 16 pixel pipelines and 6 vertex shaders. So all you needed to do was overclock your GT to Ultra speeds and you have a 6800 Ultra for $50-80 (maybe even $100) than the actual 6800 Ultra. That's one of the reasons why nVIDIA discontinued the 6800 Ultras...they just didn't sell that well because people exploited that mistake.