For example...if you have an Asus P5LD2 (which I do) you get 4 DIMM slots for DDR2 RAM = two pairs of 2 for dualchannel. If you put in one 512MB and one 1GB module and they're otherwise identical, what happens? Dualchannel is disabled, off, zip, nada, no go? Dualchannel is on but only for the first GB (512MB+512MB), the third 512MB is limited to single-channel? All goes to hell?