Is the iMac 24in (plastic white) still be supported?

You can still upgrade to OS X 10.6 Snow Leopard. You maybe able to run OS X 10.7 Lion.

But your iMac will NOT run OS X 10.8 Mt. Lion.

Mt. Lion is not supported for your model iMac.

You need, at least, the mid-2007 model to install and run OS X 10.8 Mountain Lion.

Note that you need to install the maximum RAM memory for your model iMac.

Newer re-iterations of OS X use more more CPU, GPU, RAM and HD resources.

And you need to know if you are still running older PowerPC versions of software, these will continue to work if you upgrade to OS X Snow Leopard when you install a PowerPC emulator called Rosetta.

If you can upgrade to OS X Lion, those older apps will no longer work and will have to update or upgrade to newer version of that software.

These model iMacs are listed as vintage.

Not sure about if there are parts still available.

When a model is listed as obsolete, this means there is no longer any support from Apple and no outlet for parts.

You would need to take your iMac to Apple or an Apple Authorizes repair center to see if they can replace the PSU or not.

"Vintage" means that support and parts are no longer available except in the State of California. So unless Kristosimo is in California, s/he will not be able to get support from Apple for this iMac, though a service center may still have parts available. If not, the service center will not be able to order them from Apple.

Regards.