Put simply, a receiver has two main functions: firstly, it amplifies the sound so it can be fed to your speakers, and secondly, it allows you to select the audio and video you wish to watch. You can use it to switch from a DVD to free-to-air TV, for example, with the touch of a single button.
It's well worth paying extra for a good surround system to go with your new flatscreen TV. But if you're looking at the, then you needn't get a AU$4000 receiver to go with it. I'd only spend up to a maximum of twice what the front stereo speakers are worth, as a rough guide.
If you want features, then the complementaryreceiver would work perfectly, and will only set you back half of what the TV cost. You also get a remote control that will work the TV as well. But if that's also a bit rich for your blood, then try for under a grand.
Speakers, are of course, another discussion entirely -- but it pays to shop around and listen to as many pairs as your ears and budget can stand.
Buying a big TV is only half the battle and it can be a long road to finding the home theatre system you're happy with, but the resulting experience can be very rewarding. Home-cinema-in-a-box solutions may be fine for the kids room, but if you're investing in a large-screen TV it's worth budgeting for more.