the rule of thumb, there isn't?
Don't compare temps, don't try to use a socket A setup to compare heatsinks either. The biggest thing is to make sure you're running an updated bios, and your temps are "in-range" of what htey should be.
For a quick example, a t-bird at 1ghz 1.8V puts out roughly 60W of heat. Even with a Kanie Hedgehog, the cpu core will run no cooler than 20C over ambient. So you would take your ambient case temp +20C = usually in the high 40s. If your thermistor is reporting within 5C of this(usually 5C down), then your chip is running fine.
Durons/t-birds of over 1ghz are real heat "hogs". 1.2ghz users shouldn't expect their cpu to run anywhere but in the 50C range even with great case cooling.
Mike