The "best" is going to be an eight or twelve core machine, though one could argue that the extra cores only come into play in multithreaded apps like Compressor at this point. A six core MacPro will work just fine.
Current Final Cut Pro 7 doesn't care much.
It's not a 64 bit application. They will have to rewrite it.
Get an 6 or 8 core, and invest in enough ram and good storage. G-speed Q with SATA card for example. And a good archive/backup system.
That's more important than a 12 -core.
Apps like Compressor 3, After Effects CS5, Cinema 4D can really use all Cores and Memory available.
I'm curious about this because I'm in the market for a new Mac Pro as well, along with a new set of drives. I will be editing HD along with lots of graphics and tons of footage. So, I was looking at that G-SpeedQ, but I'm not too savvy on eSata and how it works, especially with FCP and After Effects. I just know that it's faster than FW800.
But, I have been to the Mac store before and none of their Macs come with built in eSata or even an option. So, I guess it's something I'll have to install on my own. Are there things to consider when purchasing a Mac Pro if I know for sure I will be using an eSata Drive like the G Speed Q? (ie Audio Hardware like the MOTU Ultralite mk3, or multiple video cards)