If you use iTunes, as mentioned, an
Airport Express (or an
Apple TV) is cheaper (£70 to £80 for AEX and £99 for ATV2). By default, it will only stream music in your iTunes Library, but you can get software called
Airfoil ($25 or roughly £16) which will allow you to stream any sound from your PC or Mac. If you have an iPhone / iPod Touch / iPad, your music can be controlled through the
Remote app.
If you don't have a wireless network already, Airport Express is all you need - the Apple TV will need an existing wireless and will also need a digital input on your amp (so you would need an external DAC to use with your Marantz), but has other obvious benefits if you can plug it into your TV. There is more than likely going to be a new Apple TV released at 6pm today (ATV3?) though so watch the WHF news page around this time if you're interested in it.
The main downside to the above is, although the Airport Express does have an analogue output so can be connected directly to your hi-fi, it's not going to sound as good as the Squeezebox - simply because the built in DAC is not going to be as good. This can be rememdied by an external DAC at some point, but adds to complexity and of course cost. Also, since it's only natively compatible with iTunes, it's more of a faff to use if your music isn't stored in there.
If your PC is close to your hi-fi, a straight wired connection would be the cheapest way of course!