In general I consider anything below 60° to be acceptable, the chips can run hotter but my recommendation is to try and stay below that.
As for measuring the temperature of your chip, depends on your motherboard. Most motherboard manufacturers make programs for their boards to view temperatures in windows. All the boards I have seen display them in the BIOS.
No. (although they might make a mobile version - I haven't checked). IIRC the Clawhammer is basically an Opteron core.
The reason for the difference in clockspeeds for the same rating is that the Clawhammer has 1Mb of cache, whereas the Newcastle has 512Kb. The exta cache supposedly makes up for the 200Mhz speed reduction. (How much difference it actually makes varies depending on the application).
I checked on Newegg, becuase I have a mobile 3000+ with the 1 MB L2 cache, with the clawhammer core, however the desktop version has the newcastle. Anyways newegg, sells both the mobile and the desktop version of the 3200+, the desktop version have the new castle core, with 512 KB of L2, while the mobile version has the clawhammer core, with the 1 MB L2 cache.
However starting at the 3400+ they all have the clawhammer core.