Heatsinks, what means what?

Just that. You start of with gamma, but I don't know which is better, Delta or Beta? Can anyone enlight me with a list from good to bad or something like that?

The designers originally planned to start the player off with the Alpha Heatsink, but then decided that to do that was too cruel, so they give you a Gamma to start off with. From worst to best, it goes:

Alpha
Beta
Gamma
Delta
Omega

Where does the Sealed Heatsink fall into compared to the others?

Bah, you're going to make me look, aren't you? *flips through .ini files*

It falls just barely underneath the Beta Heatsink, by a margin of 50 points of heat per second.