@THX1138
If you don't need 3D then I would recommend the original Cybervision 64 by Phase 5 above anything else. Sourcing one out secondhand might be a bit challenging, though, as they were last made about 8 years ago.
If you want 3D then the Cybervision 64/3D is the best value for money gfx card, especially secondhand. Just make sure you source out one made by Phase 5 rather than DCE as there have been reports of DCE boards dying quickly after purchase.
The Picasso IV is a nice card, but people are still asking exorbitant prices for new and secondhand boards.
If you want more technical information regarding the gfx cards for the A1500/2000 (and anything else Amiga), then check out the Big Book of Amiga Hardware:
http://www.amiga-hardware.com/