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The only way to not lost quality (and with a 61 inch TV every reduction in quality will be easily spotted) is to save the original disc as is with no compression at all. To save some space you can remove some stuff from the disc, like all menus and extra footage, and possibly also foreign languages you are not interested in. This will save some space, but if you have a large collection of movies, 640GB will be not sufficient at all. So I suggest to consider getting a couple of 1TB disks (and probably they will be not sufficient anyway).
However, the resulting quality after a compression is related to the original movie. Some movies with high bitrate can be compressed more than others with no evident quality loss, so too bad there is only a way to know how much you can compress a movie: run some tests with various compressions and watch results in your 61 inch TV, and then choose the better compression ratio. Each movie will require such testing, so if you have many movies you must run a lot of tests for each movie, and this will be really time consuming.
Again, the more practical solution is to save the main movie only (removing foreign languages or other stuff not needed, mostly menus and other useless stuff), and buy some more HDDs.
The most space saving solution is compressing the movie with DivX/XviD or H.264 codecs, but again with a 61 inch TV it will be really difficult to not get artifacts in the video. If you have time, however, it could be worth to do some tests converting some short segments of movie compressing them and watching results on the screen. If you find this solution acceptable, then you can store much more movies on the HDD. Keep in mind that not all standalones are able to play movies compressed with H.264 codec, and also that these compressions require a lot oc CPU power, so if you don’t have a powerful enough computer to convert a single movie can require many hours.