Reviving an old thread just a bit, but wanted to comment from experience.
I have my 4100 oc'd to 4.3ghz on stock voltage, stock cooler and it rarely goes above 40C. I mostly game, but do some video editing as well in Vegas 11. I haven't pushed it any farther yet as I haven't really needed to.
Despite the constant AMD BD bashing my experience with the 4100 has been really pleasant. Easy to oc, stable, cool and runs really well.
As for your second question, the performance increase from oc'ing isn't that much. I notice more of an increase when oc'ing the gpu. But again, most of my time is spent gaming so that's where my focus is as well.
Oh yea, be sure to turn off turbo in bios.