Question : What memory can I use?

With Windows XP SP3 I need to add more memory to my computer.  Right now the Mobo is a MSI MS-6570.  I currently have a DDR 266MHz with only 512Mb of memory.  Would I just add another 512Mb stick, or could I keep that 512Mb in there and add a 1Gb stick also?  I don't know if all sticks have to have the same amount of memory available.

Answer : What memory can I use?

You could do either.

The board http://www.crucial.com/store/listparts.aspx?model=MS-6570+%28K7N2G-ILSR%29 supports up to 3 Gb of memory.

Best results come from 2 matched sticks of memory; say 2 x 1 Gb sticks or 2 x 512 Mb sticks.  You could use 3 x 1 Gb sticks to max out the memory on the board.