Samsung Galaxy S firmware codes
Well for the Vibrant, the codes are as follows; for example, KA7:

The first letter is the year: K is the eleventh letter of the alphabet, so 2011.

The second is the month: January (A = 1).

7 is the revision or build, so KA6 is older than KA7, etc.

It's obviously different for the i9000 but you should be able to find that info on XDA.  They'll also tell you which release is which version of Android, which is not in the code AFAIK.