Do yourself a favor and spend the extra bit for the digital in. Get the Z-5450's or the Z-5500's. Only real difference from what I can tell is the rear wireless speakers cost a bit more. You may eventually want the digital input. Many motherboards and sound cards and HDTV's have this premium connector now. As far as the room, it's only going to make a real difference if it's being used in a wide setup with your tv, because sitting at a computer with your speakers right in front of you, the sound isn't likely loud enough to bounce around off your walls and back into your ears. 5.1 > 2.1, a good step up regardless, as is THX certification. No little 2.1 cheapo's will compare to THX certified 5.1
I have the Z-5300's and the X-530's. The quality on the THX certified ones is slightly noticeable, and that noticeability is increased with the subs and volumes turned up.