On my HP asterope 3 motherboard, I put 2gb in one slot and 1gb in the other slot and both the bios and Vista indicated 2gb was present. I took out the 1gb and the bios and Vista still indicated that 2gb was present. I put 2gb in the now empty slot and both the bios and Vista indicated that 3gb was present. My suspicion is that 4gb in one slot--if I had one to test--would show up as 4gb in both the bios and Vista.