One thing that makes a Mac slow is too little memory for the tasks you are doing. Start a typical mix of the Applications you use at once, then use this article to analyze your memory use:
How much total RAM in the pie?
How much Green?
How much Green + Blue?
How many Pageouts since last restart?
How much swap used?
5 yrs old does little to tell us what yours is and has now...
But 2007-8 and you need FBDIMMs and should start with 16GB probably, some faster and more disk drives for system, media and projects, for scratch. Even processor upgrade.
ATI Radeon 5770
How To Install and Remove Memory Mac Pro2x2GB FBDIMM DDR2 667MHz @ $36
For best performance on Mac Pro, install memory in risers symmetrically
The Mac Pro comes with two memory riser cards, each with four DIMM slots. With a total of eight slots, you can add up to 16 GB of memory.
Note: DIMMs must be installed in pairs of equal size from the same vendor. For instance, you must not have one or three DIMMs on either riser at any time. Additionally, two DIMMs from different vendors should not be combined and used as a pair.
Other configurations of DIMM pairs are still compatible with Final Cut Studio, but may not achieve the same performance levels as when DIMM pairs are installed on both risers evenly.
Riser A contains: Riser B contains: 2 x 512MB 2 x 512MB 2 x 1GB 2 x 1GB 2 x 2GB 2 x 2GB 4 x 512MB 4 x 512MB 4 x 1GB 4 x 1GB 4 x 2GB 4 x 2GB
WD Black 1TB $93
WD Green 3TB $149 - backup
WD VR 10K 250GB $103 200MB/sec boot drive :
Those last and last and make a nice boot drive.
SSD: Samsung 830 128GB $99
Final Cut Pro X, Motion 5, Compressor 4: Graphics card compatibility