1. E4500, since your video card will be doing most of the work (and since it sounds like you're overclocking anyway).
2. That depends on what you're using it for. If you regularly tax the cpu, then yes you'll see an improvement. If you don't, then it will be less noticeable.
3. It's not, from what I can find.
4. As long as your monitor is big enough.
5. You won't see as much of an increase as going from, say, 512MB to 2GB, but you'll see an improvement. And for $17, why not?