hi,, as far as I can tell:
1. staring form model 8372A, Agnus chip is capable of generating both 50Hz and 60Hz timings. Hence an Amiga that has this chip, (provided that the game sets it correctly) can run PAL games at 50Hz and NTSC games at 60hz.
The 8372A or later is present on later revision of the A500 and the A2000, all ECS and all AGA Amigas. Note that a NTSC Amiga running PAL game will generate the so-called NTSC/50 video signal from the composite output, i.e. a 50hz image with the NTSC color component. It depends on the monitor wheteher this can be displayed or not. Conversely a PAL Amiga runs a NTSC game generate PAL/60. The signal generated through RGB port is the same, though (there is no color encoding in the RGB signal).
On Amiga with an Agnus model earlier than 8372A:
if you run a PAL game on a NTSC Amiga, you won't see the bottom 56 lines, and the game run faster than intended
in the opposite situation you see a big black border at the bottom, and the game plays slower.
I don't think there are many games/animation coded in 2 versions: usually the same code was sold, giving different results on different machine.
Note, however that sound syncronization should not be an issue: this is because Amiga audio is inherently syncronized with an internal clock which is (usually, it can be changed by switches on the motherboard) the same frequency of the display (i.e. 50 or 60Hz). The drawback is that audio will sound different: on NTSC notes have an higher pitch.
edited: more exactly, the audio on NTSC will sound a bit faster (so it will finish earlier) and notes have an higher pitch. But it should stay syncronized.
The approach used (one version for both PAL NTSC) has the drawback that an anim in NTSC runs faster (thus is shorter) but on both machines it will be smooth, since there is no repetition of frames (to adjust the 50hz anim to 60 hz) or frame skip (to adjust the 60hz anim to 50hz).
On PCs, where refresh frequency may vary more from machine to other machine, anim are often coded to adjust speed by repeating/skipping frames, and this may decrease the "smoothness" of the anim.
please excuse my horrible english! :-)
