i do love these incredibly vague questions [sarcasm]
the amount of video you can shoot is determined by the quality you shoot at combined with the actual space available on the memory. sure you could have a 32gb card, but if you have 15gb of music on it then you are half full already.
unless they changed... apple products never used to support external sdcards. the internal memory is all you have to work with so it applies to everything from os, apps, etcetera.
quality? compared to what...?
other phones? sure the quality is going to be mid to high end. i've heard there is a good camera in iphones.
however, the device is not a camcorder. it will not have the quality of a device made specifically for that job.
if you need a phone and want to shoot a video on occassion go with an iphone.
if you want to shoot video only spend your $500 on an actual camcorder.