You can do this easily
but I shrink or compress them first any of the free downloadable dvd compressing packages available will do I use dvd shrink or dvd2one a google search should give you some results, pm me if you can't find them and I'll see what I can do
Compress them to about 4.5 gb, the quality is still there and they take up less room and youcan copy them across quicker, I watch mine streamed onto a 42" plasma which is not very forgiving of poor quality feeds so I know the compressed files will still be great quality.
A couple of other things to bare in mind, experiment with the player software, the best free one is Vlc media
http://www.videolan.org/vlc/ it'll play anything you throw at it, try also decrypting your movies as .vob files they seem more compatible with things like mce2005. And if you get problems streaming then the chances are the NASbox is struggling, my NASbox has a modernish m/board and processor but more importantly it can transfer the data at UDMA100 speeds, I also run a gigabit network. Streaming normal dvd files should work fine, I have some high def ones and they can still struggle, hopefully vers 2.0 with SATA will sort that out. Good luck