The 6gb is perfectly fine and moving to 8 won't get you any difference in gaming. The single thing that would make a difference would be upgrading the GPU. Crossfiring 7770's probably isnt the best plan as it wont increase performance in a number of games. I would just snag a 7870 or a 660ti and sell your 7770.
Upgrading the RAM will not improve your gaming experiance in any way.
What you need to do is overclock your CPU slightly without adding voltage.3.2 GHZ should be achieveable without the need of buying an aftermarket cooler.
And secondly you really need a faster graphics card.
Buying another HD 7770 will give you equal performance to a HD 7870 in Far Cry 3, but not all games support Crossfire, so its best to buy a single, powerfull graphics card instead.