You get about 6dB per bit, so if we're talking about the digital domain, that gives you about 48dB of attenuation if you're playing 16-bit audio at 24-bit.
It gets quite complicated once you get out of the digital domain and start considering what the actual noise floor of your DAC is (20-bit would be very good performance, and state-of-the-art is in the 21-22 bit range) and beyond that, there's the noise floor of your amplifier, which will be higher still - most will measure closer to 16-bit performance than 20-bit.
The short version is: unless you're actually hearing a problem, it's best not to worry about it.
One thing you might be able to do is to use a lower wattage amplifier, or a passive attenuator between the DAC and amplifier so that the digital volume control is closer to 100% when it's at normal listening levels, if that's something you are concerned about.
Personally, I think these issues are over-exaggerated on a lot of audio forums.
As for Media Center's output being 32-bit when you only have a 24-bit DAC, I'm guessing that you are using an ASIO driver, as ASIO only works in 32-bit - at least that is my understanding.
There is an option under Tools > Options > Audio > Device Settings which lets you specify that the DAC only uses 24-bit data when using an ASIO output.
This should only really affect the noise shaping used, and shouldn't really have much of an audible effect whether it's enabled or not. It's more about being technically correct than being something you really need to be concerned about.
There is a bit-depth simulator available in the parametric EQ DSP which you can use to hear the effect of reducing the bit-depth at its extremes.
When you're actually playing music, you can probably reduce the bit-depth quite a lot before the effects are made audible.