You aren't tied to one or the other, a good mobo not only runs nVidia or Radeon but often runs either SLI or XFire also....my Z77 Extreme 4 can run XFire or SLI and I've run a number of combinations of both on it....personally I lean towards nVidia, and of course Intel
Don't feel your CPU choice constrains your GPU choice. Personally, I prefer AMD. I don't know why, I just do. Both cards and CPUs have their advantages and disadvantages. Intel uses much less power, but has a higher thermal limit so obviously produces much more heat. AMD has much worse performance per watt, and has a lower thermal limit so less heat will be produced. Though I've heard things about Nvidia's heat that I surely wish are not true, they have much better performance per watt than AMD. Nvidia also has CUDA, PhysX and better driver support than AMD cards do. However, AMD cards OC better and are better on the price/performance ratio. Right now the GTX 700 series is better than the Radeon 7000 series though I personally think the Radeon 9000 series is going to be pretty awesome based on rumored specs. Intel is the better performer in the CPU department right now without a doubt, but AMD is catching up. Pretty much once you get past the $175 mark AMD starts to become less and less of an option. Again, this all depends on your budget, which you haven't told us yet.