Also, there's the slight detail that you're trying to create a block that pays you and he's trying to create a block that pays him. So the coinbase transaction will be different. (And if you're both mining for the same pool, the pool manager ensures that each miner still gets a different coinbase transaction even though they pay the same address.)
yep, so plenty of variance in what everyone is hashing. In reality my total hashrate is between 460MHashes and 550MHahes, depending on if I want my two VisionTek 6870s to be 85C or 95C for many hours at night. I am very interested in water cooling them eventually, mainly because the noise level of the GPU's fans is like a hair dryer making out with a jet engine, but also because it is something I have been interested in doing anyway, even before i got into mining. I am guessing proper water cooling could really drop those temps and allow me to clock these cards much better.