I am running into the same issues with my QS.  It is slower than molasses.  As I no longer have a faster Mac, I also need to upgrade.  If you do go with a faster cpu, get one with L3 cache if you intend to use it mostly for FCP.  The new 7448 have 1mb of L2 which is even better because its on chip.  These are super pricey though.
Dual is always better for Mac Apps!  When in doubt, get a slower dual and it will be faster than a single faster chip.  If you wanna sell that dp 800, that would be perfect for me and you would have some cash to upgrade.  PM me if you are interested.

By the way, my old 1.42dp G4 was great with FCP, just about as good as my old 2.0dp G5.  And check out http://www.barefeats.com for some great QS upgrade testing of a 7800, x800xt and 9800 in a 2.0 G4 QS.  It was faster than a dp 2.0 G5 in some tests!