Increase the voltage? Max voltage for the cpu is 1.36 and the max full load temp for that cpu is 72 C. So, increase the voltage and make sure you have good airflow in the case and a good aftermarket cooler.
Liquid cooling... liquid nitrogen.... that would help. lol
Do you really need 4.2 Ghz? You have already made a 33% overclock, why do you need more.
Higher OC = Higher voltage = higher temps = faster death.
A new GPU will make a bigger difference than a 200 mhz overclock.
Depends on the resolution. The lower the resolution the more CPU power is needed. If you will be playing on 1900x 1200 (I hope I`m correct) a game that requires a good GPU you will get a bottleneck, but a smaller one than having the game on 1280x1024.
I think it would be like a 5 to 20 fps drop when , but you should be fine. The game will still be playable. Just lower the settings a little bit and save the money for a new CPU.
An E8400 is not going to bottleneck a GTX470 or a GTX480. Certain games like more cores but even a stock E8400 will handle any single GPU. Now if you start running 2 x GTX 480s in Sli then you might bottleneck some but basically an E8400 still holds it's own pretty well especially overclocked.
If an E8400 bottlenecks the so would an i3 530/540.