Srini,
One of most valuable WMS features is to transcode and stream video. I don't think NAS hardware (processor speed, memory, etc) would be good enough to transcode hi-def video on the fly, specially without compromising NAs performance. Eventually it would be possible to stream audio and pictures using DLNA; however, many NAS boxes out there already offer this feature.
More processing power should be necessary on NAS to do a decent job on video. Perhaps you can consider a possibility to create a dedicated home media server.