For your network, it really depends on the video quality that you stream. Full HD, like playing BluRay iso images over a network really requires gigabit wired, in my experience N wireless doesn't quite cut it. For DVD or lower (or compressed) quality video 100mbps wired or N wireless works well.
On the NAS question, I use "homebuilt" NAS boxes but that too depends on your definition. A nice QNAP or Synology work great, but if you are a DIY type like me you can build a very nice NAS that will stream BluRay images to HTPCs easily. My most recent build, an ASUS Z68/Intel i3-2300/Adaptec 6805 with 8x3Tb Hitachi in RAID 6 does a great job. Really depends what your needs are and how much time you want to spend on NAS maintainence, but even most onboard controllers will provide adequate performance with a fairly large storage array.