I'd actually suggest Logitech 5.1 speakers; which most computer stores should stock, or better yet, order them online and you don't have to pick them up
I saw the Logitech 680 at Harvey Norman for $400 a few weeks ago. The cheapest place in Sydney that I know for 5500 is Computer World.
Computer World might have the lowest price in Sydney and as it's speakers I don't think you'll need any customer service. But call them beforehand to make sure it's in stock, a lot of their stuff isn't.
If you have an amplifier, then no. Just that most of your theatre speakers are connected by a different wire to the amplifier than what you have on your PC. But I am assuming these are proper home theatre speakers, so you need another set for your PC (or an amplifier).
The 2 speaker sets I mentioned before have the same connectors for individual speakers, but they are connected to the sub to the control panel and then to the computer via 3.5mm or SPDIF.
best speakers 5.1 for pc is the logitech z5500 digital hands down. read any review on it and they'll rave about it. i got mine for about $490 at Secret (www.secret.com.au). Absolutely awesome.