How "ready" do I have to be to get the different endings?

I've been lead to believe that there are several endings in the game, and that getting the "best" ending hinges on how much Galactic Readiness I collect before finishing.

If I'm about to finish the game, but I'm close enough to a "better ending" milestone I might opt to play a bit longer to get it. On replays, I might also want to get a significantly worse or better ending to see something different on that playthrough.

How ready is ready enough to get each of the different endings?

The ending is affected by several factors, but with respect to Readiness, there are different 'tiers' that affect the ending outcome. Note that not all tiers are relevant to all decision trees. I.E., based on one decision, tiers may be at 1750 and 2050, and having gone the other way would place them at 1750 and 1900.

The key numbers for Readiness Rating are: 1750, 1900, 2050, 2350, 2650, 2800, 4000, 5000.