when you extract the zip the file that should come out should be a iso or a cso your not supposed to convert the zip file itself just extract it. As to whether you should use a iso or cso it depends. iso are good in one sense they take less time to load and usually never lag the con about iso is they take up a lot of space. the good thing about cso is they take up a lot less space the con is they tend to load slower and lag in some situations. My personal preference is cso since i have a lot of games and very few 4 gb memory sticks. it alls depends on what you would rather have little less games on the memory stick yet very little lag with iso's or a lot of games on a stick with some lag with cso's
1)When you rip a UMD, you get an .iso file. A .iso is like a digital copy of the CD.
2)when you compress a .iso using a program like this you get a .cso. All a .cso is is a compressed .iso. It makes it smaller.
3).zips have no place in the psp. That's a computer compression scheme that, to my knowledge, psp's don't recognize.
4)As far as "best" to use for your games, make your .iso files into .cso files. IF, for some reason, the game performs really laggy or slow, then uncompress it back to a .iso.
