The Mac Pro does have 5.1 surround sound. Just pop a DVD into the DVD drive and hook up a good set of speakers like the Z-5500's to the optical out and you'll have 5.1 surround sound. Games I'm not too sure about. Not too many Mac games support surround sound.
While you cannot replace the sound card in the Mac Pro, since the soundcard isn't a sound card but a set of chips and circuitry on the motherboard, you can add a souncard to the Mac Pro. I haven't heard of a PCI-e sound card, yet, there may be one, but I haven't seen it yet. There are however USB and firewire soundcards, like the M-Audio Sonica Theatre (which seems to be still sold but discontinued) or the Griffin Firewave.
The usual reason to add a soundcard to the Mac is not to improve the sound - the Mac has great sound - but to add various port options for sound in/out, such as the 3-color coded mini stereo jacks common for low and mid end computer speaker systems. Higher end computer speakers typically have the optical out.
Also, Creative in general does not support the Mac. They made a soundcard for the Mac back in the MacOS 9 days, but never upgraded the drivers for MacOS X.