" The memory I have is Single Sided not double. I have fitted x2 512MB dimms in A1 & B1 I don't seem to have any problems but is this working correct as Dual Channel by having the x2 DIMMS in the A1 & B1 Yellow "
Yes thats fine, you are in dual channel mode.;-)
" I have Crucial CT6464AA667 non ECC PC5300 DDR2 and in the bios its set as Enabled for ECC checking by default. Should I have this set to Disabled ? "
Under advanced > ECC configuration > DRAM ECC enabled, switch this to disabled. Press F10 to save settings and exit.