So, according to the chipset docs (Intel P35):
Single Channel: only one channel (A or B) contains any DIMMs. The memory controller only uses one channel. Slowest performance.
Dual Channel Symmetric: both channels contain the same total amount of RAM, but not necessarily made up of uniform DIMMs. The memory controller uses both channels simultaneously (reading/writing successive cache lines to alternate channels). Theoretically twice as fast as single channel. In this mode I'm not clear on if the DIMMs used have to match between channels (i.e. if channel A can have a single 2GB DIMM and channel B a pair of 1GB DIMMs) or if it matters which way round different capacity ones are fitted - I think the answer is "probably".
Dual Channel Asymmetric: both channels contain DIMMs but with different total capacities. The memory controller uses both channels sequentially (reading/writing successive cache lines to the same channel). About the same performance as single channel.
And from the motherboard docs (ASUS P5K-E):
For single-channel, use slot B1 or A1 (yes, the manual does list them in that order!). For dual-channel with two DIMMs, use slots A1 and B1. For dual-channel with four DIMMs, use all the slots.
The pair of slots nearest the CPU are channel A and the pair furthest from the CPU are channel B. The yellow slots are the first DIMMs in the corresponding channels and the black slots are the second DIMMs.
The manual claims "any excess memory from the higher-sized channel is then mapped for single-channel operation" but that's not quite what happens - rather, that results in the dual channel asymmetric mode.
I think this means that if I have two matched pairs that are the same size, then each pair should be placed by slot letter (to keep each pair within one channel - the idea is matched pairs have matched performance). But if I only have a single pair, or if I have two pairs of different sizes, then they should be placed by slot number (to keep each channel the same size).
Still, could be more complicated - this could be the bad old days of SIMMs when you had to fill multiple slots at a time!