CMSS is a rather confusingly-named feature, in no small part because Creative plasters the same name across different technologies.
The original CMSS appears to be a stereo upmix for surround speakers, nothing more. The X-Fi cards still have that as CMSS-3D Surround.
It's CMSS-3D Headphone and CMSS-3D Virtual that bring the binaural HRTF surround mixes like Aureal was using, for headphones and speakers respectively.
Weirdly enough, though, when I was fiddling around with an SB Live! 24-bit External's drivers, I set it to Headphones, turned on CMSS, and it sounded surprisingly like CMSS-3D Headphone. I'm guessing it's because all of Creative's USB audio devices, up to a certain point, use the same driver and software base that shows or hides features depending on the connected hardware.
Oh, and don't get a Titanium HD if you're planning to use it with XP, because Creative decided to eschew XP support entirely for that particular card, apparently in an effort to overhaul the drivers for Vista/Win7. Most other X-Fi cards have XP drivers, though, and I'm using an Auzentech X-Fi Prelude in my retrogaming box right now with the Daniel_K X-Fi Support Pack 2.5 driver package and no audio issues other than the volume resetting on me every now and then. (It makes for a rude surprise when using headphones directly plugged into the card.)