There is 2 slots...

The first slot is for PSV games you buy which are like memory sticks...

The second slot is for the new memory stick which you have to buy from Sony :)

Edit:

So it will cost you about 350/400 for wifi version or 400/450 for 3G version.

This is only if you buy a game and a memory stick.

+250$ for PSV / +50$ for new game / +50$ (+100$ for better memory stick) for new memory stick.