8375 are newer versions of what? What Amigas work with them?

[Ilwrath]

Unfortunately most Agnus chips have exactly the same PLCC packaging but they have a widely different electrical pinout.

Really??  Which ones are these, so we can look out for them?  It would be a shame to accidentally fry one.  All I've encountered have been pin compatible, but admittedly, I haven't gone looking for trouble, either.  ;-)

To make them usable in earlier 512k Amiga's you have to do some work, add a capacitor, cut a track, change a jumper.

Actually, they are USABLE as a direct drop-in replacement in an A500.  No mods needed.  And they provide boot-time PAL/NTSC switching without mods (rev 5 and earlier A500s won't do that otherwise).  If you want to enable the extra 512k CHIP RAM, then yes, thats how to do it.  

[Ilwrath]

There are actually 19 or so different versions of the 8375 Agnus, each of which has a different pinout.

Well, who am I to argue with Dean?  :-)  Good grief.  I thought all those revisions were minor mask changes and bug fixes.  Who knew many also had some different pinouts?  Hopefully none of those are destructive to each other.  [ie moved power pin(s).]  EEEK!