Don't know where you're shopping, but all the ones I've seen so far are plain old PCI bus. In fact, if you want one, I'll sell you one of mine for cheap. After buying a very inexpensive HDTV tuner card and finding out that I can can get all kinds of digital and HDTV programming off-the-air, I've just about lost all interest in the Theater 550 Pro. What with all the bugs and unkept promises from ATI... who needs it. Using my HDTV tuner is great. Even after transferring my recordings onto DVDs, picture quality is so much better than that overhyped Theater 550 Pro. Even recording non-HDTV content through my digital tuner is better than cable. Very clean, sharp, unsoftened. Unlike video from the 550, which tends to be soft. It's made worse by the fact that the 550 has a very limited sharpness adjustment range.
BTW... I bought my digital/HDTV tuner for less than a Theater 550 card and it came with Snapstream's BeyondTV. Signals come in via a 30 something year-old UHF antenna that the previous homeowner left in the attic... so it costs me nothing to get HD programming.
Contrary to what some have said here previously, recording HDTV isn't that big a deal. It's not even 3 times the space requirement of regular MPEG-2 but it has four times the resolution and color information (not to mention better, all-digital sound). I just picked up a 160 gigabyte serial ATA hard drive (for recording videos) for less than $50 US. That will easily take care of my HD recording.
So if you have access to over-the-air HDTV programming (a lot of people in the US do) just leapfrog plain old NTSC tuner cards. They're going to be irrelevant soon.
Jonas