Hmmm let's see - 625 lines at 25 fps = 15625 lines per second (64 uS).
So... if you used 96k sampling you would achieve a massive 96000/15625 = ~6 pixels per line (less than 3 if 48k - NB *never* use 44.1k).
In truth if you wanted to try it anyway you would be better off choosing a sample rate that is an integer multiple of the line frequency (say 93.750) that way there is an even number of samples per line.
A better approach would be to use the parallel port of an old PC (new ones don't tend to have them) which would give you a sampling rate of around 700kHz (from memory).
This is what a PAL picture looks like when played OUT through a parallel port:
http://www.youtube.com/watch?v=Sa04GhS99pw
Now capturing a frame is pretty much doing the reverse, in fact there is software available on the net that actually does just that (although for NTSC I think). I could probably find it if you are interested.
This shows how the parallel port is turned into a DAC, and turning one into an ADC would be similar.
http://www.youtube.com/watch?v=tW822Yv6b4s