There are various devices that you can use for streaming media content from the PC including the Xbox 360, PS3 as previously mentioned. There are various devices that can do video e.g. Dlink, Buffalo, AppleTV and so on. I would stick with a wired or high speed homeplugs solution for streaming video. Wireless isn't really good enough. If it's just music have a look at the Squeezebox 3, Duet and Roku Soundbridge. There are others but those are very popular. The Duet is brand new and has the wow factor of the remote. There are separate forums for these sorts of devices. I have a SB3 and it's one of the best gadgets I've ever bought. Networking on receivers tends to be found higher up the price bracket and I don't think I could put up with the small screen (or TV) to view tracks etc. That's why the SB3 is ideal, it has a great (clear) display and you can also use the PC to display it inc. artwork. I have my laptop screensaver setup to display album artwork using a plugin as the laptop is connected to an external HDD where my music is stored losslessly.
Take a look at slimdevices.com, ripcaster.co.uk and rokulabs.co.uk in addition to the forums on here. The SB would be the better choice for sound quality and the Roku if you're on a budget but it also really depends on how your music is ripped. If it's at a low bitrate then it will sound naff no matter what amp and speakers you play it through.
Richer Sounds were doing a deal on an older Yamaha AV receiver with networking (600 I think) that might be worth a look. Otherwise you're looking at Denon 3808, Pioneer LX70, Yamaha 3800. As it's not really a feature, I would look for in an AV receiver, I don't know if it's filtered down to new cheaper models. There are various new receivers coming out from the likes of Sony, Yamaha, Pioneer etc.