The HD7850 will outperform the HD7790; it's simply a stronger card. Were it to lag due to memory limitations, reducing AA will take care of it.
And, monsta, that's funny; you gave a Canadian Newegg link.
I have found that even in low AA situation, a 1GB card just does not keep up in newer games.
It runs slower in general but the drops are less and less jerky.
if that makes sense:D