Changing my Graphics Card
Right now my Mid 2007 iMac has the graphics card ATI Radeon HD 2600 Pro 256 MB. Is it possible to upgrade this graphics card to a better one with 512 MB?
Your system has the best video option for the series. The VRAM is soldered on.
This system has a PCI Express slot for the video card so it's possible to plug in a different card but you'll hit some issues, between the heat sink attachment may not line up with your choice and the needed video drivers.
Frankly, the amount of work to swap it out given the systems age I don't think it's worth the risks and the work figuring out which video card would work here.