TM;
Did you find the max capacity of your unit? If it was mine, I'd put in as much as I could.
As far as one 512 or two 256s, it doesn't really matter which route you take but if you put in one 512, leave the previous 256 installed for a total of 768 if the unit will take it. The memory configurator should have advised you how much it will take and how many slots there are. You will notice a very marked speed rise
Regards
carholme