What role does the GPU play in UE4 when compiling?

^ hi my HD 4870 plays reasonably well still to day and it plays Battlefield 3 aswell on the 40 fps side of things. Pretty ok if you ask me.

So would you say I can use it to develop 2D games or a basic 3D FPS game? should I invest in the i3 for purpose? its hobbyist / student projects really.

with that said I wonder how a haswell i3 @ 3.5 GHZ with 4 threads perform compared to a FX 6300 AMD?