Aditionnal memory to a dualchannel setup

if i buy a 2x256 matched pair of ram and use it in dual channel if i buy another 512 mb pair later on will the 2x256 pair still work in dual channel as well as the 512 being there as additional memory.

yes...... 256 dimm1, 256 dimm2 = 512 + 512 dimm3 ..... you'll still be in dual mode