My stock battery sprung a leak so I'm going to replace it with a sealed sidewase-sitting type recommended by srinath etc. My question is, how important is matching the "Ah" value of the new battery exactly to the old? My stock battery (from a 2002 GS) is 11 Ah. Does the new battery have to be exactly 12 V and 11 Ah, or can the Ah be a different value. I'm assuming Ah is "Amp-hours" but keep in mind that I'm completely ignorant of anything electrical.

it's pretty important,  yes Ah means amps/hours.  if you go smaller you run the risk of draining the batt down, and if you go bigger it may not charge completely. (let alone fit)