Two observations about your hardware system that have not been addressed:
You may want to switch to the ATI Radeon X1900XT for better Core Image performance. The 8800GT is only better for gaming, not for Pro Apps.
If you plan on finishing on the FInal Cut system (especially if you’re going to grade in Color), you should strongly consider changing the graphics card to the recommended model (the ATI X1900XT) for better performance.
Regarding RAM: IS your system configured with Apple OEM RAM, or third party RAM? If third party, who? While there are several reputable third party RAM suppliers out there, many are pretty sketchy. Flaky RAM is known to cause problems in Mac Pro systems. Also, there is an optimal configuration of memory modules detailed in Apple’s Final Cut Pro support pages:
https://docs.info.apple.com/article.html?artnum=305177
https://docs.info.apple.com/article.html?artnum=304492
What is your final delivery format? Are you doing a film out, or finishing to HD video? I’m assuming film out, as you’re working at 24fps, not 23.98. If you’re doing a film out, why edit in HD at all? Why not adopt a traditional offline SD editing workflow to generate a negative cut list? Even if you are finishing in HD, you might have better luck offlining in SD and then conforming to HD at the end of the editing process, given your storage limitations.