whats TDP and TcaseMax for?

On 90nm manufacturing process, the power leakage of transistors on individual processors differs greatly, this is why AMD implemented TCaseMax. Each CPU has a different TDP rating, here's how it works:

A program reads the TCaseMax value off the CPU, then depending on the processor (CPU type like A64, Opteron;CPU Rating like 3500+, 146) it finds the processor's Thermal Profile (If you download the "AMD Opteron™ Processor Power and Thermal Data Sheet" and look at pages 10-11 you'll see the different thermal profiles). When the thermal profile is determined, the program finds the TDP rating that corresponds to the CPU's TCaseMax value.
So in a few words, the higher the TCaseMax, the higher the TDP of a processor will be (the transistor leakage is higher), so the processor will run hotter. Several people have reported that the Opterons rated at 71C TCaseMax were pretty hot. On the other side these processors will be able to reach higher clock speeds.
That is why we see that on average processors with higher TCaseMax can usually reach higher CPU speeds when overclocked.
Every CPU type (A64, FX, X2, Opteron, DC Opteron) has a different thermal profile, that is why you see that on an FX a TCaseMax rating can be "only" 57C and it'll overclock way better than an A64 with a 57C TCaseMax rating. That is because at 57C TCaseMax, an FX has a TDP of 77W while the A64 has a TDP of 44W.

interesting - mine is 63 degs..

what temps you running @2.75?