This is the Final Cut X forum. Your post actually belongs in the Final Cut Studio forum:
That said, you don't need to convert to ProRes 4444 unless your clips include alpha information. In fact, you may not need ProRes 422 HQ. Try using plain vanilla ProRes 422. That should be a big speed improvement.
Final Cut Pro 7 runs on only one core. Compressor can run multiple cores, so you could take advantage of your dual core machine by converting your incoming footage in Compressor, not Final Cut. When done editing, export using the same settings as your sequence to create a master file, then use Compressor to compress to something like H.264 or .m2v for distribution. That won't speed up Final Cut, but may speed up your overall workflow.
Also check your realtime settings on your Timeline to make sure they are Unlimited RT and playback quality Dynamic.
Finally, FCS 3 seems to be somewhat faster under Snow Leopard: