bostonpinball.com & the archive lookup on here will be a good starting point

If you can get Centaur at a decent price GET IT! Restored games are up close to $2000 now. It's a great game from that era. One of the best Ballys.

They're going to range in price, but the most expensive is Centaur followed by Black Knight.

As Shock said, Centaur's are bumping 2K. A decent BK is 1.2K. A Xenon or Embryon around 900 to 1K. The rest are $600 to $700 depending on condition.

If any of the ones you are looking at that deviate from those prices by alot then jump on it, assuming cosmetics aren't completely blown.

I'd also rate the games in that order too...Centaur is just stunning and a good overall package. Probably not 2x the playability of the other games, but coupled with the artwork it's a great machine. BK is an all time classic and you can't go wrong. You can always sell it quickly if you tire of it. Xenon is another one with great art and sounds and the play is decent. Embryon is a widebody which may or may not suit you. Also great art. The rest are all about the same in play and desireability, IMHO...I'd give Sorcerer the edge for the rest of the pack...Basically a Firepower II but much much better. I see alot of seawitches so not that scarce. Less so the other titles.

Now if it were me, and all those titles were available from the same person, I'd buy them all Flip the ones you don't like, keep the ones you do.

Havent seen any Seawitches out here, shes on my witch list..

Can't advise you on prices but I know that Sterns are both rarer and cheaper here in the UK.
I can give you opinions though.

Seawitch and quicksilver are nice fast drop target games but my favourite of that type is Stargazer which you haven't mentioned. Cheetah is also worth a look.
Galaxy is a nice game but doesn't have the replay value of those mentioned. Freefall is a nice game but is slow and open compared to the rest. I would also consider Flight 2000 if you find one.

My favourite Sterns are Flight 2000, Lightning, Stargazer and Big Game

See here: http://www.pinballnews.com/comment/sterndesign.html

Sorcerer is a really nice game but is fairly easy for me.
Xenon is just awesome. The game play is simple but challenging but the sounds and lamp show build up the tension in a way that keeps me coming back to it.
Black Knight is great but it is a bit like Black Knight 2000 in that once you master the shots it becomes a bit tiresome-great Steve Ritchie flow.
Centaur always amazes me its a fantastic game. Great rules that can be tampered with by changing the MPU switches. 5-ball multi-ball awesome lights and sounds and it even has a touch of flow. You can set the MB so that it acts like an add-a-ball
Embryon is really nice to look at but gets kind of boring for me. Sterns Iron Maiden is better.
I don't like Cybernaut. The playfield always feels cramped, the sound is poor and all the above games are better IMHO.

You did ask

Great descriptions emo. How do you set Centaur mb to work like Add a Ball? Dip switch I assume, but do recall the name of the option?

I can't remember the settings but you can get it to add balls by lane changing and lighting all 4 in/out lane lights or by knocking down the central drops and releasing. It can make multi-balls last quite a while if you can hit the shots. The dip switch settings effect how easy it is to do.

Most of the early 80's games can be tweaked to suit the owner. Paragon is another game that can be made really interesting by checking the switch possibilities and changing the rewards you get for completing features.