Currently have nVidia GeForce 5700 Ultra (128Mb), seen a GeForce 6200 (256Mb). Both AGP, as cannot go PCI-E, as this means a new motherboard, which means a new cpu....
If you are experiencing problems with your current card then yes it is worth upgrading. If your current card does what you require then it is not worth the upgrade.
Out of all the items which make up a computer I'd say the graphics card is one of the most important. If you've been quite happy with the one you already have and its not acting up, unless the new one offers advantages you NEED over your present one I wouldn't bother. Make sure you have the latest drivers for your existing one though.