Hi djs498,
The Nevo will do everything you are looking for it to do, I have a similar setup myself Panasonic 42", Arcam AV8 / P7, Xbox360, Sky+, also have a naim hifi running a QNAP TS101 and Squeezebox to transfer music from my PC to my hifi.
I would strongly advise looking at Squeezebox or Roku products both work in a similar manor and can be controlled by the Nevo easily, likewise they are easy to setup (at least the Squeezebox is, I have less experience with Roku). Either product will communicate direct to your PC wired or Wirelessly via WiFi. However this means that your computer will need to be turned on when you want to listen to your music so that the SB or Roku can access the music on it. An alternative is to use a NAS drive such as the QNAP TS101 to store your music on which will communicate directly to the SB or Roku device without the need for your PC to be turned on (obversiously you would need to copy or transfer your music to the NAS drive). Note that SB requires a specific peice of software to communicate to and the NAS drive you purchase has to have the software pre-installed (QNAP is the only one I know with it pre-installed and I believe it doesn't come as standard so visit your local hifi store). Roku I have not connected to a NAS so if anybody uses one can they recommend a NAS for it to work with.
Check out -
www.slimdevices.com for info on SB
Alternatively, the Sonos system you mention is a great product. It offers same features that SB and Roku do but all of the info is relayed back via a remote control rather than the device itself. Unfortunately Nevo SL will not communicate to the Sonos devices and they use a closed system. Likewise I have done serveral tested between the Sonos and Squeezebox III and believe that the SB is slightly higher quality (all my music is stored as WAV, although I also tested MP3 320kbps with identical results). In saying that the one thing I prefer about the Sonos is that if you want to expand the system into other rooms around the house, I think it offers more flexability although there is very little in it.
By the way SB and Sonos of the products support the common sound for matter WAV, FLAC, ORG, Apple Loseless, MP3 etc.
One last note, if you don't mind leaving your computer turned on constantly to listen to music you can do as others do and use the Nevo to communicate to you PC direct and connect you PC to your system. I personally don't like this method as I have encounted many WiFi issues with the Nevo SL although I know several people use it very successfully. But my biggest issue is the drain it has on the battery life.
Hope this helps
Jonesy