You are fine. While 600w isn't ussually ideal for cfx, I think it would work. My bro had to 6950's wsith a 600w for a few months. There might be a slight ram bottleneck, since the settings you will be able to play at will require more than 1gb vram.
I would recomend at least 700 W preferably if you can squeeze in the extra 50 W then 750 W if you are going to crossfire. Be sure to get a PSU from a reputable brand, rosewill is good but the best are Seasonic, FSP, Corsair, PC Power and Cooling,and Antec.
Do you know the model of your PSU specifically? How many amps on the 12 volt rail? The 6850 has TDP of 130 Watts. Two of them will be 260, then add the FX 6100, which is a 95 Watt chip, then throw in 50 W for everything else and you're up to 405 Watts. So you want at least 405 /12 = ~ 34 Amps on the 12 volt rail. Does your power supply provide this?
As far as bottlenecks are concerned: At what resolution are you playing?
Well, The makers of assassin's creed aren't the best at programming for pc's, at least not for ac3, so I would assume that to be true for the others. But, I don't know if you even need another 6850. Also make sure that psu either has 4 pcie 6 pins, or molex so you can get the converters.
well just a suggestion I think you should upgrade to a diff card instead of crossfire if the next gen is out just because 1)your psu would be stretched on crossfire and 2) crossfire has a lot of issues with some games and some driver issues in general