Good questions.
First off the emulators for all game/computer machines are written for the gp32
but all u do is plug the gp32 into the usb port and download the roms u use to play on say zsnes etc.
Rom are the exact file as from the game system so all emulators would use the exact same rom unless it needed to be converted in the case of gba it have to be converted into a gba rom for a flash cart.
But here's an example.
i wanna upload 3 things to the gp32 i would open the pc link software and set the link mode on the gp32 in the bios (its really simple u just select the pc link button on the menu)
after the program loads ont he pc it connects to the gp32 then all you would do is go to the directory you want.
so you would upload
gianasis.d64 and rtype.t64 to the c64 directory
then the mp3's and jpegs to any directory.
your done

The gp32 works in the way a pc works you select the fxe file you want in this case it's frodo.fxe which is basiclly like a exe file so when u select it it loads up the emultator then u select the rom from the c64 directory.
your done

thats all you would do its very simple
if i didnt make sense then u upload the same roms u download from the internet no converting nothing.
Now with smart media cards there is a problem. they arn't supported in the industry now so you can only get 128mb MAX thats the biggest u can get but they are very cheap so its only like $20 USD to buy 1 or so.
You can only use smart media and nothing else.
Now i'll tell u the downside.
The c64 emulator is exctly like the pc version frodo (it's ported from the pc) and frodo doesnt play all games

there is a few childhood c64 games i can't play but to make sure yours would work download frodo and try it out if it works then it will work on the gp32

Also there are alot of cool homebrew games for the gp32 there is a cool loderunner clone and a great bommerman game comming out soon which looks fantactic
The movie playing is good but...you can only play specialy encoded divx files so if u wanna watch u must encode it again to 320 x 240 at a certain bitrate etc. But that is all simple and explained everyone just uses virtual dub to do it they get the settings they want then save it so each move u convert u just have to select the gp32 profile

Oh yeah it plays 15fps and almost all movies are fine at 15fps the matrix is ok but its a bit slow cause of all the fast action sometimes.
But you can overclock the gp32 to get higher frame rate which will run your AA down faster.
The mp3 players are great and there are text viewers and all that and windows like desktop apps

There are alot of apps like filemanager paint program calculator and my fav Tone dialer

u put the number in and it outputs tones from the gp32 speakers so if u play the number on the gp32 it will make the phone ring if you have the speaker against the mouth piece

I'm blabbering on but it can do alot of things and its have great pc ports like bump n jump and chickens 2 alll the dooms and rise of the triad wolfenstine 3d and soon dukenumkem 3d it seams

but i'll shutup now