I'm trying to figure out why both of my Nvidia cards prevent booting the Amiga.

With either an old TNT M64 or a fresh from the box 6200, the Amiga simply won't boot. A PC sees them with no problems.

I've seen the same result in both a Mediator 4000 and a Prometheus.

Is this a symptom of a 3.3v card in a 5v slot or what?  It's just too weird that all the other PCI cards I have seem to work, but not these Nvidias.

PCI slots and cards are either 3.3v only, 5v only, or universal. A 3.3v card won't fit into a 5v slot (unless you put it in backwards). All PC's have 5v slots, so all cards for PC's are either 5v only or universal. I don't know about the Prometheus, but I think on the Mediator they decided to not provide 3.3v on the PCI slots even though it should be present even on 5v only slots. So it is possible that the problems you are having are because the cards are expecting 3.3v and not getting it.

5V vs 3.3V PCI is not about power supply but about signal levels. A 3.3V only, non 5V tolerant card will either not work or be destroyed in a 5V slot. The other way around, a 5V card in a 3.3V board can destroy the board and other cards inserted.

Usually the notches prevent anything incompatible from being inserted - 5V cards have the notch away from the bracket, 3.3V cards on the bracket side and universals both. All Amiga PCI boards are 5V, so you'll need 5V (very old) or universal cards.

All TNT2 M64 PCI cards I could find on the 'net are either 5V or universal - but: I can remember having trouble with an M64 in an old Pentium board of mine (Asus T2P4) due to it being not PCI 2.1 compliant.

both these cards are keyed as universal, but i've seen quite a few reports of cards that claim to be universal that are 3.3v only.

Any other explanations as to why it wouldn't boot?

qed

Both these cards are keyed as universal, but I've seen quite a few reports of cards that claim to be universal that are 3.3v only.

Any other explanations as to why it wouldn't boot?

Many universal cards still require the 3.3v power supply lines to work. The Radeon 9250 cards Elbox sells are modified to include a 3.3v regulator on board to supply the needed 3.3v.

You can see pictures of it here : http://www.a1k.org/forum/showpost.php?p=264387&postcount=31