what set up do you have?
If it's a dual system then you will need the first two memory slots to equal the last, ie 256 + 256 + 512.
If however it's just an ordinary setup then it doesn't matter what mix you have as long as your board supports 3 memory modules.
Check that you buy similar ram and that they are rated the same, as they will always run at the slowest module.
Other than that you will see an improvement from 512 to 1gb.