5.1 is only useful with recording in dolby digital 5.1. each channel is 'discrete', meaning it has separate, unique 'information' (ie: centre channel plays different music to front L or R, and rears are different again). you need a dolby digital 5.1 processor obviously.
if you're not interested in buying recordings in DVD format, then just stick with stereo. i have a fullblown 5.1 setup, and i do not enjoy playing normal stereo CDs via 5.1; it sounds poor compared with simple stereo. on the other hand, genuine 5.1 recordings sound awesome, but i have few of these and rarely play them.
for that HU, you will need a matching pioneer dolby digital processor if you want 5.1 sound. but they aren't available in australia as far as i know. the DEQ-P9 is NOT what you want; it's not a dolby digital processor. in the end, it will be expensive, and unless you're a hardcore 5.1 listener, probably not worth pursuing. but if you want to play music DVDs or watch DVD movies, then worth doing.