Good luck getting your 15-pin DSUB analog connector (on your powerbook) to interface with the digital ADC connector on the Apple studio display!
Unfortunately, the two are not compatible and there is no converter that I know of that will make it work.
There are a couple of options, however. One is to get a PCMCIA card that has a DVI-out (like
Margi's Display-to-Go adapter. With this, you can get
Dr. Bott's DVIator to adapt DVI to ADC.
Another option is to get an external PCI enclosure (looks like a small computer minitower... don't remember who makes them). Then, get a PCI card with an ADC adapter like a Radeon card.
The least expensive option is to get another company's LCD that is compatible with an analog input. The video quality isn't as good as Apple's digital solution, but it'll save you a bundle. I can drive an external monitor at 1280x1024 in millions of colors with a decent refresh rate while using my laptop's screen as a primary monitor.
Hope this helps!