Doom 3 will support 5.1 sound. I would recommend at least a Geforce 3 graphics card and 1ghz+ processor.
The only way to get dynamic Dolby digital 5.1 sound in your games is from an X-box, or Pc using a motherboard that has the nvidia nforce or nforce 2 chipset (do a search for nforce on google). The nforce audio is only possible if you get an nforce based motherboard. If you plan to get a pc together in future and nforce 2 may be a good bet (we're still waiting for them to be released).
Philips acoustic edge is meant to be a good soundcard. Go to
http://www.3dsoundsurge.com/
for soundcard reviews.