Resurrecting this thread...
I think that vid chip makers really missed the boat on not supporting PCI-X, and I'll tell you why.
The video card industry has supported legacy PCI, which I appreciate, but IMO it's a bit ridiculous to see a GeForce 9500 GT with 1 GB of RAM with a legacy PCI interface.
Many of the buyers of legacy PCI cards are using old server boards, like I am. These comps were the most powerful thing going in their day, e.g. dual 3 GHz Xeons, so are still adequate for many uses today, even for a server 7 or 8 years old. These can be built very cheaply now from eBay. Their one down side which can't be overcome is the lack of a decent video option. USB 2 you can add, GigaNIC you can add, SATA you can add. The architecture of many of these boards included multiple PCI buses, so with judicious choices of what peripheral goes in what slot everything could be made to work very well together, very fast. Unfortunately, most PCs only have one PCI bus to work with which usually shares with NIC or sound card, or both - but I know a large number of legacy PCI vid card buyers are using old servers, who would benefit a lot from a PCI-X vid card, judging by such things as PCI vid card reviews on vender sites.
PCI-X is backwards compatible with PCI - the unused pins will simply not be used, and the card need be no longer than a ordinary 32 bit PCI vid card, so there is really no need that two different cards be made for PCI and PCI-X, as long as they follow the universal spec.
But let's look at the numbers. Going by memory, PCI 32 bit has a data rate of 133 MB/s, PCI-X @ 133 MHz has about 1 GB/s. AGP had a max data rate of about 3 GB/s, and PCI-e x16 has a data rate of 4 GB/s (please correct me if I'm wrong, though the numbers are just for rough comparison).
So, yes, PCI-X doesn't really compare with PCI-e or even AGP - but it is still 7 and 1/2 times faster than regular PCI! Given the performance I have seen with just a regular PCI vid card (on it's own bus with no other competing peripherals) this would give a HUGE performance boost - enough to make the bus quite usable for most games, or for flawless Aero performance, which is a huge plus.
I think if one of the vid chip manufacturers wised up and sold a Vista compatible chip with PCI-X support, he would sell a TON of them - but I also doubt if we'll ever see it, unfortunately - the market mentality for regular 32 bit legacy PCI seems just too entrenched, which is just sad - sad for the chip manufacturers, and sad for all the potential customers.