How do you know this? what makes a GPU OpenCl capable?
Driver support and programable shaders. Most likely unified shader model cards such as the ATi Radeon X1900 + and the Nvidea Geforce 9800(though i believe Nvidea were much later on the unified shaders than ATi) and the Intel GMA 4XXX GPU's.
The other option would be shader model 2 cards starting with the Geforce 6XXX(The FX range are to poorly preforming at SM 2 instructions and missed a lot of the instruction set) and the ATI X6xx.