Later this year X-OOM will be releasing Media Centre for the Wii which will apparantly, once installed, let you use the Wi-Fi on the Wii to stream everything on your PC to your Wii/Tv. It should support MP3s, WMAs, MPEG4s, DivX and XviD and will cost £29.99. While I am quite interested in this product I will be holding off buying it until I see some reviews as I think it will be glitchy, though I hope it works great.
The other possible problem is that recently I have been watching more and more HD stuff through my media PC and I am not sure if the Wii would be able to handle it, I know the XBMC has limited HD capabilities, the Wi-Fi on the Wii certainly wont have enough bandwidth for HD. I also think the only LAN option is through USB so this could possibly rule out HD. However on the bright side it is only 30 notes and could make for a nice cheap media centre if you already on the Wii