It's really going to depend on what you're doing with the graphics. As soon as you start emulating basic shaders for effects on the CPU by comparing and writing to a screens worth of pixels you'll see where the bottleneck ends up.
On the other hand if you're wanting to make something with the graphical complication of Mario or Mega Man ( purely sprites - no effects - lighting and other things maybe done with alpha blending ) then you might find it makes sense to stick to working with the CPU.
On the other hand if you're wanting to make something with the graphical complication of Mario or Mega Man ( purely sprites - no effects - lighting and other things maybe done with alpha blending ) then you might find it makes sense to stick to working with the CPU.