Bifta - you are using the XT PE BIOS arent you and not the PRO BIOS. I could'nt get anywhere near the speeds i get now with the PRO bios, only after i tried the XT BIOS was i able to OC much.
But your Watercooling heating up that much sounds - worrying - im stock cooling on my X800.
Just got the new
AtiTool as well (0.0.23 Beta 4) which supports Find Max Mem / Core on the X800, and its looking like 546 is rock on the memory so far - so i might get a higher score still.
Not even tried find max core yet.