graphics card for emachines t1840

Depends on your budget and what you need the graphics card to do.

You can put a graphics card in any computer that has the correct ports on the motherboard. Take a look at the device manager and see what your motherboard is. Then you can figure out what ports it has, whether it be pci, pci-e. Also, as emosun said it iall depends on what you want to do.

Cheers!