Because the detail settings aren't high/max. You listed games with medium settings. Good if that's the best you can afford, but most of the time we are talking about 1080 and high/max settings. If you are aiming for that, the 6770 will disappoint. If you are ok with low/med settings then you'll be fine.
Would my Athlon II x4 740(3.2ghz) record games easily?
I don't see that CPU recording well while at the same time giving you high frame rates in the games. I second Nvidia for your GPU. Make sure it supports shadowplay.