I beleive there are no PCI-X slots on the new machines.
As for video cards goes, from my experience and research there's not much point to getting the higher end video card unless you do a lot of work with Motion or Shake etc. I recently upgraded my 9650 to the ati 800 and didn't notice much difference in editing in Final Cut Pro. I would recommend getting a good card to drive 2 DVI displays and then invest in a HD or SDI card or something of that nature to drive a Colour monitor instead of spending all your money on a higher end video card.
He has a point! I bought the ATI 800xt mac edition and only saw a difference in Maya , motion. Only a little bit in FCP. If you not doing anything with 3D or motion graphics, I would stick with getting more RAM.
The apple website has a 2.7ghz G5 with PCI-X slots (here apple.com, go to the store, pick the G5, in the upper right is a link to the machine with the PCI-X slots). Are these necessary?