What Studio Display is it? Is it a CRT or an LCD?

If its an LCD, you're out of luck, they all use ADC, which is impossible to use on anything pre-PCI. If its a CRT, you might be in luck. If its the later one with the completely see-through rear case, you're still out of luck, as they also use ADC. If its the translucent blue and white or graphite one, you are in luck - they use an SVGA connector, and can be connected to your IIsi with a Mac -> VGA adaptor.