The middle number changes when we run out of stock and make more CDs. Thus the appearance of 2.7 means that all of our 2.6 CDs have been sold. The 2.7 CD includes all of the 2.6.x patches.
The last number changes when we release a patch of some sort.
If GIB somehow doesn't work on your machine, we'll happily take it back. If there's something about GIB that you don't like, we'll work with you to fix it. If you give us a reasonable shot at helping you but you're still unhappy, we'll generally take it back but reserve the right not to in individual cases.
GIB's performance can also be degraded by the presence of virus checkers that examine modified areas of memory. This is because GIB writes to many segments of memory in analyzing any particular position. If GIB is playing slowly and you have such a virus checker, you'll need to turn it off before starting GIB.