Get an ATI 4650 or a nVidia 8400GS/9400GT. They are OpenGL 2.x or better and CS4 supports them.
Thanks for the advice.
I bought an ATI AH 4650 works a treat with quick zooming but shows up how slow the rest of the system is Athlon 3000+ 2.5GB with PATA drive. The original SATA went pop and the 320GB replacement wouldn't work. The BIOS seems to have a maximum SATA size less than this, I don't know what it is but will start to dig into the motherboard and BIOS and see if a second SATA drive as a scratch disk will speed things up. I can always use the SATA drive if I fit a new motherboard.