Realtime FX workaround
It's important to note that RT is designed for G4's. The minimum seems to be 500Mhz in this processor class.
You probably already know that you can preview in real time in QuickView, which is found in your
Tools menu; the result is a tab in your Tool Bench Window in "Three Up" layout. Even many power users of Final Cut Pro tend to overlook the amazing abilities in the "QuickView" tool. Switching to a lower render quality - say medium level - makes a dramatic increase in speed in many instances.
That said,
in a recent MacRumors thread discussing the immenent release of Final Cut Pro 4 there were some real genius insights, including
a tutorial that might solve your question (credit to
crassusad44 - thanks much). I highly recommend you visit that thread and bask in the sunlight of our MacRumors community knowledge
I found also that the Manuals that accompany Final Cut Pro define well the specific hardware requirements necessary for genuine RT effects. Check Volume 4 of your manuals, page 189.