You won't need a new motherboard. You can always upgrade the i3 to an i5 or even an i7, in either locked or unlocked (k branded) cpu depending on the motherboard designation, only z boards and some few b/h boards will currently allow OC. So there is that option.
Yes, the cpu is important. Every game made is a program, it is comprised of lines of code. This gets interpreted by the cpu, then the graphics get sent to gpu while audio etc is processed by the cpu. If you have a slow, weak cpu, it'll hurt the gpus performance, and a strong gpu just gets under utilized.
Ram will help considerably. Windows itself uses @1.5Gb-2Gb of ram to run, and most cpu intensive games like unity, bf4, Skyrim and WoW can use 4-5Gb. I have 16Gb, and when running heavily modded skyrim, I've seen it use 57% ram, or about 9Gb. So an upgrade to 8Gb will help.
As far as Ac Unity goes, don't feel bad about poor fps. It's the game engine, not your pc. The Ac series use different game engines, and alone are pretty good, but unity had to cobble together 4 different engines into 1, making it quite hard on the cpu and gpu to keep up with it. Honestly, if you are not running an overclockable i5/i7 with at least the gpu power of a gtx970, Unity is pretty much going to suck.
So.
Option: upgrade cpu to i5/i7 maybe k for OC (requires a decent cpu cooler)
Option: upgrade ram to 8Gb
Option: OC your i3 if possible.
While not as powerful as a 970, the 770 is still a respectable gpu and will still get decent fps in Unity although not at the highest detail settings.