Can i put any graphics card in my computer? 10 points?

my 2008 imac’s graphics card is a radeon 2600 which is old and bad and it uses Pcl express so i was wondering could i put any new graphics card that fits? (all imac graphic cards even new ones) would fit. i want to put a really powerful radeon like in new macs, can i do this? do i have to upgrade the power supply
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