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i have this sff HP, HP Elite Desk 800 G1 pc and i want to replace its stock power supply from 350 watts to a higher because ive heard that if i plug in a single fan GPU onto it, the psu will fail. besides, i am also planning to extract its MoBo and put it into a bigger case for more room. is these possible??

Usually, it is difficult to upgrade the PSU. You will have to make sure to find one with the right connectors. You could most definitely remove the motherboard, if you can take it apart. If you find a bigger case, PSU upgrade will be significantly easier.