There were very few ever made as the market for such an item was small back in the day. Guide to LC PDS Video Cards
Low end mac has listed a few options but I do not see them being any better than the existing video system on the 6300CD. If you want better video, I assume you would have to look at another system.
Laptop: Late 2006 2ghz Black, Pismo 400, 1400c x 3, 180, 170
Duo: 230, 280c, 2300c x 2, mini dock x 2, Duo Dock II
Desktop: LC, G4 Cube, IIfx, Umax C600, PM 8600, G4 1.4ghz DA, Mac Mini 2011 i7 2.5ghz