I have a Power Macintosh 5400/180 that I've spent a fair bit of time screwing around with. I know being an all-in-one machine they're generally not regarded as particularly upgradable. At least, that's what I thought, until I found all those spare Power Mac motherboards lying around at what is essentially a local computer junk shop.
Specifically, I threw in the 200MHz board from what I think was a PM 6400 or something like that. 256k cache + about 128mb of RAM. That board's in and working flawlessly - the extra 20 MHz wasn't a significant upgrade or anything, but it's a little speedier, at least. I was surprised to see that the desktop boards are plug and play with the all in ones.
However, that 250 MHz board from a 6500 I've got really makes me squirm - Each and every time I've tried to put it into the 5400, it WORKS, at least for a while. If you do anything graphics intensive (I was trying to see how the 250MHz CPU + ATI Rage2 would run the original Unreal Tournament demo as a test) the system freezes.
I even tried grafting a larger CPU cooler with fan onto the chip. No such luck. I'm suspecting the issue is overheating, but now I'm thinking less that it's the CPU overheating and more the GPU - perhaps the ventilation in the case isn't good enough compared to the desktop units?
I digress, though, since I've now turned someone's 'check out my sweet new macs' thread into a 'why doesn't my unrelated mac work the way i want to' thread.
Dreamcast270mhz, I would suggest you keep the 7200 and/or the 6600 and sell off the rest, unless you have a particular attachment to them. The 7200 is a robust and versatile machine, and I suspect your 6600 is actually a 6500, since I can't find a 6600 listed anywhere. If I'm wrong...well, I'm curious. The 250MHz processor is in line with the 6500, and I'm betting if you pop the case, you'll see that the CPU's got a cute little fan on it, and a nice ATi stamp on the graphics chip. That computer, if it is a 6500, is worth keeping for sure.
If you can find a G3, or even better a G4, processor daughter card, you're really in business. You might even be able to get an early version of OS X running on it, depending on who you ask and how you go about it.