Jack:
At this point I can tell you that pretty much all PCI-E cards from
ATi in the market are reasonably well supported by the latest driver
(8.8.25) in Linux. You can expect pretty much the same performance
difference compared with an AGP card with and without Direct Rendering
on your PCI-E. However PCI-E implementation is two fold dependant: It
depends on the driver end and it depends on the kernel end to provide
support for the interface on the system bus (though I think as with
AGPGART, the graphics driver is capable of providing this too, like the
internal or external AGP driver in fglrx).
My other comments regarding performance were mainly when you compare the
Windows Vs Linux performance of ATi hardware, which obviously is due to
drivers... I'm confident, though, that in a reasonable amount of time,
now that ATi has started to gather momentum, ATi will have pretty good
drivers with impressive IQ and paralleled performance...
