first off your card is nVidia which is SLI, Not crossfire (ATI)
So if you really do have a crossfire motherboard then you cannot have SLI 9400GT's as you would need an SLI capable motherboard.
Secondly, with SLI the card need to be the same. So you would need to get another 9400GT 512MB.
Thirdly, The memory is NOT doubled or added. For example: 2 x 9400GT 512MB Does NOT equal 1GB og video memory, But you would have 2 x GPU's.
Lastly, Yes if you have 1 card that is overclocked higher than another card whilst running SLI or Crossfire the overclocked card will be underclocked to the slower cards clocks.