I plugged my 5800's 8gb card straight in 5130 to shut the mouth of se rep who was telling nokia dont support high capacity card and it become slow and bla bla bla. It worked perfectly . Startup was consideranly slower but after that no slowness or whatever . Same for the music player , scanning was slow(with ~1500 songs) but then it was ok . Dont worry about card capacity as long as you put good branded original card.
I'm using 5130XM with 4Gb card...has 1200 songs and around 50 videos in mp4 format.
Here are my observations...
After restarting the phone media player will update its library and this process takes some time because of more number of music files.
Folder browsing speed is not affected by the number of sub-folders or music files in a particular folder.
However it is affected if the files inside are java files like games or applications(.jar). The more java files you have in a folder the more time it takes to open the folder. But i think it is same in case of 1Gb card as well.
About the audio quality....its awesome
the default headphone is crap...you will have to buy good quality headphones to take full advantage of the superb music quality it produces.
According to GSMArena 5130 is has better audio quality than 5800XM and most SE walkmans other than W995(pretty much same). I dont know to what extend its true.