Depends.
I just downloaded Fearless and that was like 600 MB(I think, not at my comp right now)
So on a 2 gig, you could put 3-4 on it.
Theres alot more effort needed to put movies on your PSP(you gotta convert the DVD's to PSP format, but first you gotta rip the DVDs or download a rip.) but it can all be done for free.
I got this program that does shit for me. It takes a while(I let it run overnight) but it gets the job done.