with a room that size either pc speakers or a ht system would work.
some better pc speakers may sound better than some low end ht speakers but in general i would say ht speakers are capable of much better sound if you are willing to shell out a little extra cash.
pc speakers are plug and play while ht speakers would require either adapter cables, an amp or a receiver.
a spdif output (optical) can carry the signal from your pc to a receiver or other similar device. if you plan on using pc speakers or adapting using 3.5mm to composite cables then you will want standard pc speaker 3.5mm ports on the soundcard. 5.1 requires 3 (LF/RF, C, LR/RR channels).