Ahh, a excellent question. Due to the fact I only have the 16gb iPad I model, I am limited to what films I can put on the device, so I started askig the same question. How to stream to my iPad from a pc or server, or a NAS box.
You have two options.
VLCStreamer & AirVideo.
VLCStreamer is a excellent apps, you install the vlcstreamer helper on your video storerage system, (in my case I use a apple x server) and your set. Open vlsctreamer app on iPad and you will see you streaming server - then its a matter of navigating around until you locate your movies, tv shows etc. Press play. Depending on your network speed, and processing power of the server, you can expect any media to begin streaming in about 30 seconds.
Option two is airvideo
This app is amazing, it uses your computer or server as a video converting streamer, similar to vlcstreamer, air video allows you to navigate to directories for media files in a similar way, however instead of playing the file on the server and sending your the feed, it converts on the fly, your iPad downloads the movie converted into mp4 only seconds before you watch that part.
Vlcstreamer is free, and will ask for a donation, well worth any donation given.
Airvideo comes as a lite version and a full version for $4.99 on the app store. The lite version hinders your file navigation only providing a small view of the available files.
Somwith 3 gb of free space on my iPad I can have thousands of media files available to me in a jiffy. Hope this helps you
Practically seamless both options are the best way to stream media to your iPad with the need to spend hundreds of dollars on offical apple systems such as apple tv.