for a? psp3k? psp2k? other than dat? 1st hand? 2nd hand?

what is the different between a class 4 & 6 memory card? transfer speed or the speed of reading?
does mame emulator can be play in psp?

It's pretty much up to you. Both are similar to each other, but the 3k has an awesome LCD, and it has a few other features packed into it. The 2k unit is also just as good as the PSP3k. You ought to try both the models, and see which design and model suits you best. Hacking wise, both (assuming that the 2k models sold are packed with a Pandora proof motherboard), are similar to each other. You cannot get a full permanent custom firmware on it, but you can instead use a temporary custom firmware that functions just as good.

Possible or not, I don't know.

But if you wish to get permanent custom firmware on your PSP, it would be best to just go and buy an already hacked PSP2000 model which has a permanent custom firmware like 5.00M33 and 4.01m33 for example. This is because slim models sold these days are all equipped with the Pandora proof motherboard resulting it in being unable to have a permanent custom firmware installed on it the Pandora's way.