1) from wwdc latest information, Mac pro user will have only 3 Apple official option OpenCL that is : GeForce 8800 GT, GeForce GT 120 and Radeon 4870
2) but I think GTX285 and FX4800 will support too. (At least final ver. of Snow leopard)
3) Assume you want many core the best selection from 1) & 2) is ATI Radeon 4870 because it has 800 stream processor. GTX 285 have 240
4) Assume you want memory speed the best selection from 1) & 2) is ATI Radeon 4870 again because it GDDR5 (but i can't confidentally confirm it when 4870 have lower memory speed and lower memory bandwidth)
2) but I think GTX285 and FX4800 will support too. (At least final ver. of Snow leopard)
3) Assume you want many core the best selection from 1) & 2) is ATI Radeon 4870 because it has 800 stream processor. GTX 285 have 240
4) Assume you want memory speed the best selection from 1) & 2) is ATI Radeon 4870 again because it GDDR5 (but i can't confidentally confirm it when 4870 have lower memory speed and lower memory bandwidth)