It appears that if you have any modern processor, then it will run in asymmetric dual-channel, which means the first 16GB will have dual-channel bandwidth and the last 8GB single-channel, so there is no downside to 24GB.
Intel has bragged about their Flex Memory Technology since the i915 chipset for Pentium 4 in 2004 that allows for this.
With AMD it is less official but it appears that since AMD's 7xx northbridges for AM3 have operated RAM in unganged mode (that is, two independent 64-bit channels), benchmarks have shown that you do indeed get dual-channel bandwidth when mixing RAM sizes this way too, until the matched sizes run out.