Hi Franko.
I have an Amiga 1200 with a Mediator SX board, but I also have a Apollo 1240 card instead of a Blizzard PPC, so what I write here might not be entirely relevant to your setup. But since nobody has replied to your post yet, I thought that I could give it a go.
I seem to recall that you have a custom made tower for your A1200, and if you want to install a mediator board in there, the first thing to do is to check if you have enough room inside your tower. The mediator comes with an adapter which is connected between the A1200 motherboard and the accelerator card, and the mediator itself is then connected to the adapter. This means that the total lenght (or height if you will) of the motherboard + adapter + accelerator is increased by ca 1cm. In other words, you should make sure that the height inside your tower is at least 43cm.
The standard way of powering up the motherboard on towerized A1200's has usually been to cut the cable from the original Amiga PSU and then connect the wires of the cable to an ATX PSU, but with the mediator you can forget about all this, because all you have to do is to connect the ATX PSU mb connector to the mediator, and then the A1200 motherboard + accelerator will be powered by the mediator instead! Another cool thing (in my opinion) with the latest mediator models is that it allows the user to turn off the Amiga in software (by using the PowerOff command).
As for attaching PCI cards to the mediator, it is recommended that the Ethernet card (if any) are mounted in the top slot, other cards should be mounted below that and the GFX card should be the last one (at the bottom), this is based upon problems I personally have encountered and solutions I have found on the net.
The best GFX card that will work with the mediator seems currently to be the Radeon 9200 type of cards, I personally use a Voodoo 3000 card, but I'm considering a upgrade to a Radeon card instead. Setting up the GFX card on the Amiga may be a bit confusing for beginners since it takes a while getting used to how the program for configuring screenmodes works, but once you understand this, all that is required are some patience since fine-tuning the modes can take a while.
When it comes to software compability, there shouldn't be any problems with programs that allows the user to select a screenmode in a system-friendly way, but unfortunately programs that lacks this feature + the vast majority of games can't be opened on a GFX card screen. Some of these programs can however be forced to open on a GFX card screen by using mode promotion software. All in all, I would recommend that you find a solution which allows you to switch between the Amigas native video output and the GFX card output. Personally I have a LCD TV with VGA input which I use for my Amiga, this allows me to view GFX card screens via VGA and Amiga native screens via SCART.
If you want a PCI sound card as well, you will have to find a way of combining the paula output with the sound card output, this can probably be done by using an external sound mixer, but the easiest way is to connect a cable from the Amigas audio ports to the sound cards Line In or AUX port, then you can connect Line Out on the sound card to whatever speakers/amplifier you are using.
If you use a PPC based MP3 decoder, you should probably be able to play MP3 files in full 16 bit quality while still being able to do other stuff on your Amiga as well. As for myself, I'm not so lucky since although I can theoretically play MP3 files with good quality, the actual decoding is still too slow.
I hope that the stuff written above is of any help to you, and I'm more than willing to answer any questions you or others have regarding the mediator.
