By moving the ram into the other slot you just dissabled dual channel. Dual channel requires you to have one stick on each channel, ususaly the channels are offset on the motherboard so it goes like this.
A-B-A-B. That means channel A is slots 1 and 3, and channel B is slots 2 and 4.
If you want to upgrade and still have dual channel I suggest dumping BOTH of the 512 sticks and geting a pair of 1gb sticks and inserting them in slot 1 and 3. Or you could still keep the 512 sticks and put them in slots 2 and 4, and have 3gb of ram in dual channel.
If it only shows up as 512 when you have the sticks in 1 and 3, you need to clean the ram slots with a can of compressed air, and then insert the ram cards carefuly and remove them carefuly a fue times to clean the contacts.
If you ever put ram in a computer and it doesn't show up, there is something wrong with the slot or the ram. In your case it showed up in a different slot, so you probably just need to clean it, however, it might also mean that the ram you own is not dual channel compatable, in which case I would chuck it when you get the new ram and just get 2 sticks of 1gb.