It's soldered on to the mainboard, so unfortunately you're going to keep using that so long as you have that eMac. However, Macs (even eMacs) tend to have a fairly high resale value, so you could theoretically upgrade to a whole new eMac for relatively little.
My opinion is that unless you're a serious gamer or 3D content creator, the video an eMac comes with will probably be fine for most (if not all) of the life of the system.