> 1.5 volts used in most AGP 4x and 8x cards.

8x speed is AGP 3.0 and thus 0.8 V. Luckily, most AGP 3.0 cards are backwards compatible with AGP 2.0 (1.5 V) and some even with AGP 1.0 (3.3 V).

> Obviously, computers using the older 2X standard and higher voltage levels could not
> be made to work with newer video cards.

Depends on the specific card, see above and see Divinity's posting ("few AGP cards 9600, 9700, 9800 are usable").

> If I'm not mistaken the old Matrox G450 and G550 cards are some of the few
> cards designed to work in any AGP slot (regardless of voltage).

Yes. While speed-wise conforming to AGP 2.0 they are also compatible with both AGP 1.0 and AGP 3.0.

> Does anyone have an idea what the voltage requirements of currently supported
> cards are?

Currently supported Radeons are either AGP 2.0 (1.5 V, like R(V)100, R(V)200, RV250) or AGP 3.0 (0.8 V, like RV280). Most AGP 2.0 cards are backwards compatible to AGP 1.0 (3.3 V). For AGP 3.0 cards see above. Supported Voodoos are all AGP 1.0.

> what does the Pegasos supply to its AGP slot?

See your question (and answers) from half a year ago:

https://morph.zone/modules/newbb_plus/viewtopic.php?topic_id=6837&forum=11#70287

> if the older Powermacs only support 2x AGP, is that so bad?

It's just that many AGP 3.0 cards won't work in an AGP 1.0 slot.