quads run hot (just a fact of life)
G0 (core) stepping max temp is 71C (I think this is what you have, based on idle temps)
B3 (core) stepping max temp is 62.2
cpu-z should display one of those two (G0 or B3) for the revision # (revision # = core stepping in cpu-z)
I run a Q8200 (M1 stepping) which has a max of 71.2C
OCed from 2.4 to 3.0 (1.145 Vcore)
my load temps are about 60...I can't really tell unless I stick a temp probe in there (pcprobe II and bios gives me 36, realtemp gives me 66-54-60-62, coretemp gives me 76-64-70-72)
Realtemp gives my lowest idle core temp as 43C (then again, it gives me that when I'm running 2.4 @ 0.9875 Vcore...so I have my doubts)Ok, so after setting the correct TJmax for realtemp my reported temps dropped 20C (from the ones above)...I don't know what to believe anymore

In short: 58C load for a Q6600 is good, you could push it a bit more if you wanted to