I'm troubleshooting a friends system

He said he heard a pop and then smelled something burning

Computer wouldn't start after that

I hooked my power supply to his mobo and only the led light on the mobo turns on...

Off/on don't work

No fan spin from anything

Capacitor fell out of power supply... question is this

Can a power supply send a surge to the mobo and fry it if the capacitor blew up in the supply?

Or can the mobo not working be something else?

Also
The mobo never showed the right amount of memory from the start... could using only single stick of 512MB pc266 memory in the dual channel slot make the mobo misread the amount?

It is possible that a damaged power supply can damage a motherboard.

If the system was working prior to the power supply problem, I would say it is unlikely that something else is causing the problem unless when you were working in the case you unseated the memory or the video card. One thing to try would be to reseat all the components.

Another thing to try would be to disconnect all molex connectors and case fans and see if you can POST using less power. Your power supply may not be adequate for that system's needs.

The BIOS not showing the correct amount of RAM would not likely be caused by a single stick in a dual channel pair of slots. However, some motherboards have trouble reading some RAM (usually based on the number of chips on the DIMM). You might want to check the manufacturer's website to see if the RAM has known issues with that board, or if they recommend specific brands of RAM.

If it was a power surge that killed the PSU, then I can testify that this can and does kill other components in the system. I have seen systems that have killed even the keyboard (along with hard drive, floppy, cd, motherboard, cpu - everything)

If I put these questionable pieces back onto a new mobo and psu... will the system even boot with damaged goods?

if i put these questionable pieces back onto a new mobo and psu... will the sytem even boot with damaged goods?

The only components you absolutely need to boot (POST) a computer are a power supply, motherboard, memory, processor and video card. If you can boot with those in place, add other devices one at a time--testing after each one.

Of course, you could put it all back together on the new mobo and power supply, and use the above steps only if it doesn't boot with everything in place.