First of all, let's not confuse PCI-X and PCI express (or PCI-e). Those are 2 competely different things that are completely incompatible with each other from a hardware point of view. PCI-X has been around longer than PCI express and is basically an extension of PCI that's mainly found in servers and perhaps a few high end workstations. I think it's unlikely we'll ever see a PCI-X DVB card. About the only kind of cards you see in PCI-X are high end raid controllers and multi channel gigabit or 10 gigabit ehternet cards as these are the kind of cards you might use in a server.
As for the additional bandwidth available with either PCI-X or PCIe, this would be of no use whatsoever to a DVB card as they don't even come anywhere close to utilizing the available bandwidth of a regular old 32bit 33Mhz PCI slot. Don't forget there's a big difference betweeen Megabits and MegaBytes. So even if you had a "fat" 40Mbit HD stream that would still only be about 5 MegaBytes/sec and would only be a small portion of the 133 MegaBytes/sec of bandwidth a regular old 32bit 33Mhz PCI bus has to offer. So, while we may eventually see PCIe DVB cards, it will only be because that will eventually be the future standard and the kind of expansion slots most motherboards will have and it will not be because there's any need for additional bandwidth. Anyway, regular PCI is still going to be around quite a while and I don't expect we'll be seeing PCIe DVB cards anytime soon. Frankly what I'd be more interested in seeing would be more DVB tuners with either a USB 2.0, firewire, or gigabit ethernet interface.