Any idea regarding the usual storage capacity of a magnetic swipe card ( like the ones that are used in ATM's ) ?
(PS : Tried googling but didn't get anything satisfactory and point on)
From http://en.wikipedia.org/wiki/Magnetic_stripe, it appears that the card is composed of three tracks (as previously mentioned by Pax), and Tracks 1 and 3 have a bit density of 210 bits/inch, while Track 2 has a bit density of 75 bits per inch.
If the strip is 3.25" long, then it can theoretically store 1607 bits, with 682 on each of Tracks 1 and 3, and 243 on Track 2.