All Mac Pros have PCI-E (PCI Express) slots, as do later model 2005 G5 towers. The installed PCI-E graphic cards typically won’t work however if swapped between different Mac models. For example, of the four graphic card options for 2008 Mac Pros, only one will work in a 2009 Mac Pro, and none will work in a 2010 Mac Pro.

To address this confusion, Apple has posted a KnowledgeBase article listing which PCI-E graphic cards will work in each Mac.