trying to upgrade memory have 2 banks installed put second bank in matched pair from crucial (512mb) computer will not start. have swaped pair same result will only work with one pair installed.
What is going on!
:cry:

I am assuming you have 4 memory banks, and that you're matching pairs up by the colors of the banks. Yes, if you were to have two 512MBs in two similarly colored banks, you'd be activating Dual Channel mode.

So, you say having 2Gb... two pairs... doesn't start up the machine. Usually this means the motherboard can't handle the 2Gb, and if you could provide the motherboard model and manufacturer, I could try and find the general restrictions that apply to it.

I'd also like to point out some motherboard options create difficulties for having two pairs of Dual Channel. However, I don't know your board, and therefore don't know its BIOS.

mother board asus p4r800-vm 256 mb X2 in first slot (Blue) trying to put 256X2 in to black slots to give a total of 1gb if that makes sence