The Prometheus is (currently) incapable of "bus mastering". The manufacturer (Matay) always promised to fix that soon and that it could be fixed by a "firmware upgrade".
I don't know Matay's current state (e.g. if they're still active, supporting the Amiga etc.), but the Prometheus is now on sale for 2-3 years and the problem is not fixed yet. There's a new american distributor who makes promising claims, but I wouldn't purchase one before that problem is definitely solved.
Without bus mastering, the only cards you'll ever be able to use with the Prometheus are gfx cards (mainly the Voodoo3/4/5 cards, perhaps OS4's Radeon driver will be released for OS3), and 10mbit NICs. No sound cards, no TV cards, no 100mbit NICs, no USB.
The Mediator from Elbox does cost quite some money. It's capable of busmastering though, that's why it supports sound cards, 100 mbit NICs (IIRC) and TV cards. Elbox are said to have pretty good support for their customers.
The bad thing about Elbox and the Mediator is that Elbox are paranoid wannabe-monopolists. It has been more or less proven that one of their drivers contains code that will destroy your hard disk if you're trying to pirate it (which is *illegal*, no matter what you think about piracy). Developers only get access to the development kit after signing an NDA, and Elbox are blocking attempts to make "OpenPCI" (a PCI standard that wants to create one single API for all busboards, Amithlon and MorphOS) work with the Mediators. In other words: you are *completely*
depending on Elbox when it comes to drivers.
Elbox offer the "Spider USB" card, which is just a standard PCI-USB card, bundled with their own USB driver (more expensive than the USB-PCI cards you can buy at your local retailer of course). However, they ship this card with an *unregistered* version of the USB stack "Poseidon". The Author stopped accepting Registrations from Elbox users, because he felt the company was playing dirty games with him (their advertsiment does not inform customers that they'll have to register the USB stack seperately, they blame the USB stack for problems of their drivers etc.). I *think* you can Register Poseidon again, but I'm not sure about that. Check this first if you need USB.