Question: Is it possible to use a Soundscape Mixtreme soundcard in one of the newer PCI slots that deliver 3.3 volts?

Answer: Yes, in fact answers to many questions regarding PCI slot compatibility can be found here:

What are the advantages of using a PCX[np] card..?

Question: What are the advantages of using a Digigram PCX card [with np or pcx driver] instead of a VX card or a Multimedia sound card?

Answer: The PCX cards have an onboard processor for the digital signal (DSP chip) that relieves the CPU from having to process en/decoding of MPEG audiomaterial.

The Digas modules implement this feature as follows:  Multitrack/EasyTrack NP use the DSP only when recording in Recordpage/Reporterbox.

MultiRec with NP-driver has the same logic as MTE.

Edit-Station NP implements the DSP in playback.

Rec_np.ocx uses the DSP in recording and playback.

These applications only make use of the DSP when processing MPEG material, because Linear does not require encoding or decoding.