Don't touch anything from the GUI, just create UserData/advancedsettings.xml with the correct content.
Its all about 1 single setting, cachemembuffersize in advancedsettings.xml. Noting more, nothing to do from the interface.
You can have a bigger cache on your RAM. <cachemembuffersize>5242880</cachemembuffersize> (Or the value you like)
OR Set to 0, it will cache to hard drive, No size limit beside FATX and hard drive space limitation.. "<cachemembuffersize>0</cachemembuffersize>"
You certainly cannot use a 64MB cache on RAM! 64MB is the amount of RAM the Xbox have, you cant use all the RAM for cache. ^^
5242880 Bytes equals to 5 Megabytes.. (64 Megabytes = 67108864 Bytes)
In a nutshell, when streaming content from online, all its doing is "downloading" a "file", but you can watch that file at the same time it download.
So this "file" need to be stored somewhere till it get processed so it can be display on your screen.
And thats what cachemembuffersize is all about.
Now the DVDplayer cache is what's already been processed and ready to be show on your screen.
This doesn't really need any adjustment, and certainly wont help anything with online streaming.
(mplayer is not actively supported anymore, use /report with DVDplayer please)