I have an MSI motherboard with Athlon 64 (3200+) (socket 939). I have no option to change this


I need to buy new RAM (DDR 400) because some of the old RAM melted.
I can buy 2x512 or 2x1024. Someone told me that my computer might not be able to use 2x1024 effectively, and may run slower than if I just get the 2x512.
Is that true or not?
Answers much appreciated.
Regards
Krigl