1. Some Win 7 computers have that offer going right now, you can upgrade (if it is really an upgrade at all) for $15 or something when 8 hits. With buy-you're-own OEM discs it's probably not happening.
2. As a person who has owned both the i5-2500k and i7-2700k, the answer is (for most people) very little. The 3rd gen equivalent chips i5-3570k and i7-3770k are a very small bit faster and have somewhat better graphics integrated, if you don't use a video card. However if you don't have special needs all of these processors are closely related and will perform pretty close to one another in everyday use. And all can be overclocked quite a bit.
You can compare benchmarks from the bottom of that range (2500k) to the top (3770k) and you can get a more precise look HERE
. Some of those benchmarks are designed to measure heavily threaded applications like 7-zip, but it's unlikely you're sitting around compressing files all day.
People like me nit up the differences a lot, but for Joe Computer User, these are really all exceptional high-end parts. Most likely a regular (non-k) i5 chip would smoke everything you'll want to do for years to come, and the leftover cash could go to something you'd appreciate more. Like a SSD, better video card, better display, a new winter coat, whatever.