Seems like they should give you the 6800 at no charge. I know that AMD does a greater than or equal too in performance on RMAs. I sent in a 1600+ and ended up getting a 2000+ back because they where out of stock.
512megs of ram ram with HQ mode will hurt you. Also run the time demo more than once because the first time you run it you will be paging the Hard Drive alot. Also vsync will usually show a much lower framerate if you have it enabled (but also a much more realistic number)
I was comming from a 9700pro onto an XFX 6800GT, and it was a nightmare until I formatted. I think it was the darn Catalyst drivers messing everything up. FYI a botched driver install will also make the temperature stuff not show up.