some say that bulldozer is future proof
Anyone who uses a term like "future proof" in regards to computers, should have everything they say taken with a grain of salt. There is no such thing as future proof. One can buy slightly better parts than they need at the time for the sake of a little longevity, but any CPU and Video card you go with is pretty much going to be junk in 4-5 years, no matter how nice it is today.
The only things in your system that are truly future proof are your case, power supply, optical drive, and to some extent your hard drive. Faster hard drives may come out, but most likely you can still use the older one as a secondary.
Now back to the topic at hand. Between the CPUs you've mentioned and the one we suggested, the Phenom II 965 has the best chance of guarenteeing you 3 to 4 years of relatively smooth gameplay (of course depending on what video card you pair it with), and if its $16 more at your local shop its worth every penny more over the FX-4100. Although, they are also $109 from Newegg.com currently.
The Athlon II basically is the same CPU as the Phenom II, however the Phenom II has one more core than the x3 450, and the P II also has an L3 cache, which the Athlon II does not. Depending on the game you're playing, this lacking cache can have anywhere from a 5% to a 20% hit in gaming performance. Buy the better product and always remember "you get what you pay for".
As far as Windows 8, even if there is an improvement, its not expected to be greater than 5-10 percent. Assuming Windows8 improves anything, this would merely make the FX-4100 as good as the Phenom II 965, not better than it.