If the 667 MHz memory is what is currently in your computer, that is what will be reported, and it works just fine in the 2008 Mac Pro, however the Mac Pro supports 800 MHz and it will run faster in the event you decided to upgrade for memory read/writes (which can be a bottle neck and you should consider upgrading if speed is that important to you.)
I recently changed my RAM from 667 MHz to 800 MHz on my Mac Pro with the following results reported by NovaBench:
677 MHz speeds (1 card only):
4096 MB System RAM (Score: 104)
- RAM Speed: 2028 MB/s
677 MHz speeds (2 cards populated):
8192 MB System RAM (Score: 146)
- RAM Speed: 2132 MB/s
800 Mhz speeds (2 cards populated):
16384 MB System RAM (Score: 189)
- RAM Speed: 2343 MB/s
I've seen speeds of up to to 2600 MB/s as well with 800 MHz RAM. I need to fiddle with the RAM and see why I am not getting those speeds right now, as I had been earlier.
What ever the slowest memory is in the computer is what the Mac will run all the memory at. So if you have 667 MHz memory and mix them, all of it will run at 667 MHz. Replacing the memory is the only way to get the speed increase.