There was another post with this issue, it was the DirectX setting in the game or the system. See what the settings are in the game for DirectX it uses. Aside from that, that is not a very fast video card and AC Unity has pretty high system requirements, I am thinking that the main issue is that it's too slow of a system for that game, not an issue with the memory usage.
Minimum requirements for the game:Video Card: NVIDIA GeForce GTX 680 or AMD Radeon HD 7970 (2 GB VRAM)
Both of those cards are well over the Radeon 7520g.