What is the importance of L3 cache? Is it just like what L2 cache was for 775?
Do new processors share cache to all cores? Or is a single core supposed to use it's limited memory, like it's brothers (separate)?
I'm not exactly sure how L3 works, but what I do know is it's not that important for consumer applications. The Athlon II's were able to maintain ~95% of their Phenom II brothers despite having no L3 cache whatsoever, whereas the Phenom II's typically had 6MB of L3, and the chips were otherwise identical. That fact alone influenced my decision for the Athlon II x4 630 on my first build over the pricier Phenom II's.
There are multiple levels of cache. The highest level currently is L3, and that is typically shared between all cores. Each core has dedicated lower level caches (ie L2, L1). I wouldn't pay too much attention and focuse more on benchmarks that are similar to applications you will be using.