directx 10 is slated to give more than twice the performance of directx 9.
And yes the directx 10 cards will run on a PCI express slot.
there is no way the're going to change that.
Also if you want to reap the benefits you will have to change ur operating system to MS VISTA.
Directx 10 will not be available as an additional download..you have to get VISTA to get it.