There are 3 reaons why I wanted to do such a thing:
1. Amiga 3000 power supplies are getting harder to find and when you do they tend to be pricey. An atx case/psu is usually cheaper than buying an Amiga psu alone.
2. Adding a pci bus card (e.g. Prometheus or Mediator) will not fit in a standard A3000D case (not if you want to keep the lid closed)
3. It's just cool.

Before I go into the details I must thank a few people:

Miles Jenkins (http://homepages.ihug.co.nz/~miles.j/) who gave the information I needed to start this little project.
Patrick Norton and Leo Laporte (www.thescreensavers.com) who opened my eyes to the power of a Dremel.
My father, whose patience and curiosity made this a great project for the both of us to enjoy.
and My mother, for not killing me and my father because we made the house a mess.

O.K. O.K. enough of the sentimentality, onwards and upwards!
 
 

To start, we had to decide on a case. A full tower has plenty of room, but unfortunately we don't have enough room in our office. So that means we had to go to a mid-size case. There are plenty and we live halfway between 3 Fry's (one in Houston and 2 in the DFW area). The only downside to that is if we buy the wrong item, its another 4-7 hour drive to Fry's. So we decided on an Antec KS-282 case ($79.95).This case had the room we wanted (18.1x17.3x7.9). This would give us plenty of spare room for the Amiga motherboard. That was our number one priority at the time. It has 3 5.25 bays and 3 3.5int. bays.

        The next step was the most nerve wracking for me (I'm extremely paranoid of ruining my Amiga) and that was the wiring. Using the guide at  http://www-und.ida.liu.se/~daner964/buildingdescription.html We wired the atx psu to the Amiga motherboard before we took any other steps just to confirm that the psu was working correctly. Everything powered up nicely so we crimped the ends of the wires from both the psu and the connector from the motherboard with quick connects that way should the need arise in the future to replace the psu, we don't have to mess with electrical tape.
 

before

Next the backplate of the case had to go.As you can see, as with most atx cases the backplate was not designed to handle an Amiga motherboard. Not a problem! After removing the top and sides, we used a drill to remove the rivets that held it in place. Now we could see how to place the motherboard.

after

Hmmm, the bottommost 5.25 bay seemed to be in the way, so it had to be removed. We really didn't want to lose the space though. The floppy would not fit in the proper space provided for it, so we decided to only cut away that portion of the 5.25 bay that was in the motherboards way. (I'm not using the fast slot now, and probably never will). We used the leftover portion of that bay to mount the floppy drive.

Now onto the next problem - the power switch. Atx cases do not use a push button switch like the 3000 uses. No problem! We ignored the powerswitch from the case and used a rocker switch from Radio Shack (part no. 275-712) (if you want a more simpler look you can use radio shack part no.275-1565b) and mounted that switch next to the floppy drive.

Well, that's over. Next comes the motherboard.
 

We placed the motherboard inside the case (we really didn't like the idea of keeping it in the old case and placing them both inside), and marked where the holes should be. Using a drill (7/64 bit), we made the holes in the case for mounting. We then used a tap/die set to put the threads in those holes. The motherboard then installed with no problem.

Now comes the most tedious part - making the new backplate.
 

We made a pattern of the back of the Amiga case on card stock. We placed this pattern on the back of the motherboard. We went to a sheet metal shop and bought a piece of sheet metal 18x7 (26 gauge). Then drilling holes for the rivets and mounting screws would go, we mounted it to the back of the case. On the inside we marked out the area we had to work with. We marked an area for the power supply for it had to be rotated 90 degrees. Then using a Dremel tool with cutting discs we cut out the spaces we needed for the motherboard and psu. We also cut holes for the keyboard.mouse,and joystick. We used Belkin cables (part no.F2N209-06 and F2N009-06) to extend the keyboard and mouse ports*. If you notice, we didn't include a spot for card slots. I have none at this moment, and plan on getting either a Mediator or Prometheus card. Then we will put in the appropriate spaces.

    We cut the connector from the amiga case for the power and hd leds and crimped them together with the leds from the case. Then we assembled the pieces together and made sure everything still worked. It did! Now the only thing left to do was to spruce it up with a new paint job and this is the final result.
 
 

Well, here it is. I used Krylon gloss black, along with fluorescent green. This looks really great under a black light. I made a case badge for it but I have a laser printer, so it really doesn't look as good as it could. I hope you found some of this information helpful. Good luck on your tower conversion!

On the other side of the case I added some artwork. I went to Spencers Gifts and found a really nice blacklight poster. I trimmed it to fit and then mounted it to the case with glue. (The colors are a little washed out due to the flash).

UPDATE!
 

Well I finally bought a Mediator and went to install it. First thing we did was to cut a new backplane because the backplane we were using was cut too close to the bottom of the case to allow the mediator to fit properly (my bad!). We also did not put in holes for the mouse/joystick ports in this new backplane since we were going to add the card cage. We had kept the card cage from the original case for this purpose. In order to properly place the card cage, we removed the motherboard from the case, attached the Mediator to it and then attached the card cage to the Mediator by placing a pci card into the the top and bottom slots to see where it would fit against the new backplane. You should have at least one slot at the top of the card cage that will not be used by the Mediator. Of course this will depend on the card cage you use (the cage that came on the original case had seven slots). This empty slot will come in handy later.  Then we drew an outline on the inside of the backplane so we knew where to cut. Applied the dremel and there you have it. (note: you may have to trim the card cage a little, (at the side closest to the motherboard), to allow proper fit. )

 

Now we attached the backplane back to the case, reinstalled the motherboard and Mediator. We dropped the mouse/keybaord cables down to the uppermost slot in the card cage (see I told you it would be handy), and attached them there. We also found a much better way to attach the keyboard extension cable to the backplane. We used a 3/4 inch NEER nm/seu cable connector (c-750   49660) (available at Lowes).  I hope you find this new information helpful.