Well, there are iMac G5's and Power Mac G5's. An iMac G5 is a big white iPod on steroids with a big screen. A Power Mac G5 is a big metal box with handles and no screen.
:/
If you have an iMac, I can tell you fairly confidently that you won't be replacing the video card. If you have a Power Mac, it's possible, but I'd highly recommend that you have someone do it for you.
If it's an iMac and the video card is truly screwed, then your computer is basically worthless. If it's a Power Mac, well, there's not much difference in the resale value. One repair and it's fixed.
But tell us what leads you to think that the video card is messed up. Also, please clarify what sort of Mac you have, as it makes leagues of difference.