Before the 7800's came out, the X850 XT PE was the king. It owned the 6800 Ultra and everything else nVIDIA had. The only way you could beat one was if you could afford SLI, and that was when SLI hadn't fully matured yet, so as a result it was still a bit buggy and unreliable.
Yeah, you can't mod the 7800's whatsoever. The 7800 GT's pipelines are completely locked down.