If you can afford it and your monitor is 1080p, get the 2GB. The 1GB has been shown to completely fail in some games when the settings are turned up even in 1080p. This is assuming you haven't bought it yet.
I have two sapphire 7850's and they've both clocked up to 1.2 GHZ. I use Tweak, as for some reason, MSI afterburner doesn't allow me to change the voltage even with all the cfg changes and what not. They are pretty beastly overclockers, many people get 50% overclocks on the core.