I think what Omega is trying to say is that currently the 8600 can not run High end current games at full settings even in DX-9, so if you try to add all the extra things the card has to handle in dx10, it simply isnt going to be able to do all that well.
And once you have to turn down the settings on a dx9 game, its not going to be able to handle all that dx10 is going to throw at it.
Sorry im not explaining this very well, not feeling to well today. Basically if you can't run the graphic settings of dx10 at full and have to turn the settings down your not taking full advantage of what the game has to offer. And if the 8600 cant handle the current dx10 games all that well, the 8400 doesnt stand a chance at gaming very well.
Idk im sick, take it as you will.