Hello.
If you're not adept at major upgrades or computer repair, I wouldn't recommend tampering with the CPUs. The 5150s are fine for this system.
Save up for that new Octo if you want more CPU.
What you might want to do is work with the memory, hard drives and video card.
I see you have 5GB ram. That means you are probably using 4 or 6 slots in an arrangement that is probably slowing you down by dropping the system out of quad-channel mode. Best results are by either using similar modules in all 8 slots or the first 2 slots on each card. For example, I noticed a huge performance advantage by going from 9GB in 6 slots to 8GB in 4 slots.
Final Cut itself can't use more than 4GB but Snow Leopard sure can! My suggestion is for 4GB or 8GB in 4 slots or 8GB, 16GB in 8. Don't use the original 2x512mb sticks.
Hard Drives: you didn't specify what your drive arrangement is.
You want at least one scratch disk and a capture disk. Separate drives, not merely a separate partition. You want to set Final Cut to Capture to a disk other than the system disk and render to a separate disk also. At the very least get a second internal drive for capture and scratch. Having 4 drives total wouldn't hurt as you can have OSX on 1, Capture on 2, Video Render on 3 and Audio Render on 4. Partitions don't matter here. More partitions won't make it faster (or slower).
Video Card: The best video card for the Mac Pro 1.1 for our purposes is the now discontinued 8800GT. Fortunately, some of the 8800GT cards made for PCs can be modified so as to duplicate the Apple supplied card. If you lack the resources or nerve to do it yourself, many people on eBay sell already modded 8800GT cards. The 8800GT is the only card that will work properly in the MP1.1 that supports this Open CL thing in Snow Leopard.
Of course the old ATI x1900XT is quite good but very noisy.
ATI once released a "PC/Mac" variant of the HD3870, but those thing go begging. Apparently there's been some problems with that card.
The various ATI cards made for the newer Mac Pros reportedly work in our MP1.1s but are not supported or recommended by Apple. Use these at your own risk. They probably won't hurt anything but might have odd problems.
You could get a 4870 and if it doesn't work out save it for that Octo...
That 1.1 is still a very strong system.
Good Luck,
Keri