I have a similar problem but with a 64gb collection of music and only a 32gb DAP (the Cowon i9+ which I adore). If you have the DX50 then really just start saving for a 128gb card. But in the meantime this is what I do to make the most of a DAP situation with not enough memory space;
1) Using winamp or some other music software, try and rate songs in yer current music collection. You wouldn't need to rate every song, just pick an album and give the tracks you like 4 or 5 stars etc
2) That will take some time, but now you will have some songs with 4-5 star ratings. Using Winamp or some other music software, create a massive playlist whereby only the 4-5 star rated tracks are included. This shouldn't be too hard, a good music software like winamp should allow you to sort yer music based on rating, right click>add to playlist, or highlight all drag n' drop etc.
3) save the playlist.
4) No dl and install the program AmoK Playlist Copy. Add the playlist you have saved to the program and what it will do will generate the files that are in that playlist to another new location (thereby duplicating the 4-5 star rated songs to a new location of yer choice).
5) now connect yer DX50 and copy the new 'best of' collection to yer DAP. It won't be as extensive as yer main collection but at least u'll find yerself hitting the 'skip' button less frequently.
6) as u dl and buy new albums, rate the songs, add the tracks you like to the best of collection and the entire album to yer main collection, then use some kinda sync software to sync yer best of (with its new material) to yer DX50 (when it's connected to pc). You get the idea.
That's what I do. It's quite an interesting process. Of the 64gb collection I have, my 'best of' comes to only 20gb. Heaps of space still left on my 32gb DAP. Sometimes I add 3 star rated tracks as well etc.