Hey guys, I need some help.

I recently bought a 1080p monitor, and as nice as it is, my 4850 really struggles with every game I put at it, except for CSS or TF2. Now my questions..

1) Will a 5850/5870 bottleneck my E8400 4250Mhz?
2) Would it be worth upgrading to an i7 920 ontop of that, and ocing that to 4.0 GHZ...will that give me a decent FPS boost when paired with a 5850/5870 when compared my current overclocked CPU? What I mean is

E8400 4.25 ghz + 5850/5870
Versus
i7 920 4.0 ghz + 5850/5870 = How much more of a boost?

3) I hear you can OC a 5850 to 5870 speeds, so would it be better just to buy a 5850 and OC it to kind of get near 5870 speeds?
4) How about 5870 overclocking? Would it be worth more to buy that for 100 bucks more and oc that instead?

Thanks.