In a current CPU the TDP is only vaguely related to power consumption, and mostly in that low TDP parts have their max performance throttled to hit a specific max TDP number--at idle they're going to be close to the same. In an old opteron the power management features are much less advanced, and the idle power consumption in a typical "mostly idle" fw configuration is going to be much, much higher than a relatively cheap processor with a more modern core. And not just the CPU--the older chipset, especially for a rackmount server, is going to be much more power hungry than something new. If you're paying california power rates I'd probably just buy a newer box rather than fiddle with the current one. At lower power rates pulling one 4386 and leaving the system alone is the most practical approach, because the break even time for buying anything else will be long.
