There are many reasons for wanting to do this type of modification - it lets you drop the CPU from the top of the case, which is usually hotter, as hot air rises, and it also turns your GFX card the "right way" up, so that you can see that amazing logo on the heatsink.
A "true" BTX case will retain the power supply at the top of the case, with just the CPU right at the bottom. This is better, but when starting out with a standard ATX case, it is a lot more work. I haven't actually had the oppertunity to finish this case off yet, as it was pressed into service for my brother's retro Celeron rig. However I will show you the basics, in the hope that you can use this as a base for other modifications, like windows, painting and such.
The first step is to disassemble your ATX case until you just have the chassis, without the side panels.
http://i3.tinypic.com/w06ow0.jpg
At this point you should note how the top and bottom panels fit together, as you will have to undo them. If you are lucky, your case will be screwed together, if you are normal, you will have rivets (which you can drill out), and if you are unlucky it is just one single bent piece of metal (in which case you should give up
).
Next, you can remove the top and adjourn to the workbench.
http://i3.tinypic.com/w06pvm.jpg
Now, remove the rear panel from the bottom panel of the case, for me I did this by drilling out the rivets that held it in place (if you have screws you have to undo them). I also de-riveted the support bar that holds the motherboard plate in place, and took the motherboard plate off (it was removable) this made it easier to see how things would line up.
http://i3.tinypic.com/w06q1v.jpg
Once you have done this, take the back panel, and turn it so that it is 180* to what it was before. Then you can reattach it to the bottom plate. For me this caused some issues, as the mounting holes for top and bottom plates where not in the same place, so I had to drill out extra holes in the case, to rivet them together At this point you should also reattach the motherboard support arm.
Once you have riveted or screwed the back panel back on, you can put the top back on too, in my case I had to drill some more extra holes, to get the screws in. You should find that the side panels fit perfectly, as both the top and bottom plates are in the same positions they where to start with.
http://i3.tinypic.com/w06uq9.jpg
Magic!
