Expansion slot for different moods of music? This sounds like a very bad idea. Flash memory is very expensive and I can't imagine you would have the money to buy a different card for each mood.
This is not the way they are intended to be used. You can swap music in and out as often as you like from your computer, so there is no need to have different cards.
There are some players coming out with expansion slots. This is more for future proofing, allowing you to buy a much higher capacity card when prices come down in the future.
Take the money you were going to waste on the memory cards and spend it on a top quality player and headphones.
Flash based players are the smallest and lightest and have no moving parts, though they have less capacity than hard disk players.
In flash players I would recommend iAudio at the moment, for the sound quality:
U2 - good sound quality, reasonably priced
U3 - amazing sound, plays video, expensive
Alternatively you could get a hard disk based player with enough room to store his entire collection. Again, I would look at iAudio or Creative.