Pureoverclock.com has completed an article titled: Sapphire X800 GTO Ultimate

I was already expecting this card to overclock well but my expectations were blown away. This card reached 617 MHz on the core and 1146 MHz on the memory. The memory clock is good, but the core clock is mind blowing. It's a 54% overclock! I'm not sure if I've just been lucky or if this is indicative of what you can expect on these cards, so I tested another X800 GTO I had to hand. The other card, which is not a Sapphire card reached 570 MHz on the core and 1110 MHz on the memory, so though the Sapphire is probably an above average clocker, it seems the high 500's are highly likely for all.

Sapphire X800 GTO Ultimate Review