I was a complete novice to moving hardware in computers and was scared to death, but knew I had to make the move if I was going to be adding new equipment to my PC (gamer needed video card and sound card!). BUT it was SO easy. All the parts of hardware in the 300s are easily transferred to another box without special brackets. Gateway makes somewhat proprietary systems, but all the main pieces are standard. They just figured out how to jam it all in a little box and somehow forgot to tell the sales and tech people that the 300s does indeed have an AGP slot and/but if you add anything to the PC it's going to get dangerously hot.
The only caveat here is to make sure you're grounded well. I managed not to short anything out and all I was doing was working on a clean table with clean hands, handling all cards and parts by their edges and occasionally touching the screw of the faceplate of a nearby outlet to ground out any charge my body might be holding.
I suggest getting a box with a larger power supply. This allows for more expansion (more power outlets) should you wish to add future hardware. In fact by jumping to a 300 watt power supply I had to bundle the remaining outlets up and tuck them into an empty place in the case, but I was glad they were there when I added another HD and a floppy (the floppy was so I could flash the BIOS. Gateway support doesn't provide you with a floppyless solution for BIOS updates. Black to match - $10 NewEgg.com) drive.
The motherboard is ATX compatible so be sure to get an "ATX compatible" case. Everything else should carry over rather easily. One thing I had to do was configure a "blank" (the piece of metal that covers your standard output ports (ie. sound, parallel port, monitor)) to fit the 300s' goofy output layout, but that's not too hard with some patience. The plates are very thin and easy to work with. In fact I left the outputs bare with no problem until I could fashion a proper cover plate. I suggest you do this as it adds stiffness to the mobo, keeps out dust, and keeps everything lined up nicely.
Another thing I ran into was the power button on the 300s and it’s little light have a six or eight pin (I’d have to open the side of my pc to look) connector whereas my new case’s power button only had two pins for the power, two for the reset and two for two different LEDs. By trial and error I figured out which two pairs of pins of the six or eight was power and hooked one of the LEDs into another pair to show the case was powered up just for the heck of it. The reset and additional light I didn't hook up. I called support to ask what the pin assignment was for the motherboard connection, but it was outside their realm of knowledge and responsibility I guess.
I didn't have to buy any other products to move over the internals. The case I purchased came with a little ziplock of screws and washers and such (I've still got a slew of extras) to secure the HD, CD-ROM, etc. securely in place and the screw that holds the motherboard down transferred nicely to the new case.
Was it worth it yes, I saw a SIGNIFICANT temperature decrease (15 degreesish) inside my box using Intel's temperature monitor program I mention above somewhere. Of course, I bought a case that has five fans in it, but I'm not a stickler for noise. I was able to add an additional HD, floppy, I will be adding a DVD ROM burner when I sort out how that all works, and normal size PCI and AGP cards fit my motherboard now.
I found no pitfalls. Just a box that was cooler, more space for add-ons, and room for full size cards!