I don't think you will get awesome framerates wit hthe g 620 as the cpu even the i3 surpasses it on performance but it should work in your system maybe low to medium details.
The G620 is about as powerful as an old Core 2 Duo, but the G620+650 Ti should definitely be fine for your resolution at medium settings or above (depending how CPU intensive the game is)
AC4 runs horribly so you're most likely going to have to play at lower settings
g620 with hd 7770 can play most games @ high settings @720p. In games like crysis3,BF4 you may have to turn settings down to medium. Donot worry to much about cpu unless if you put a high end gpu