Umm...I don't mean to ask a question where I should be answering them, but.... :S
If you want to run Final Cut Pro, and probably Shake too, then wouldn't you need a Macintosh? Because as far as I know, you can't coax Final Cut Pro to run on Windows, unless you "install" Mac OS X on your Windows System, which is quite a risky job in itself.
Otherwise, if you want to run other compositing/editing/3d softwares, then go for a quad, or better yet wait for a nehalem. Read it up on the internet.
And puhleeaze, stay away from these gaming GFX cards which won't be that helpful to you in intensive editing. Buy a quadro IF you can afford it, or ask around in some studios if they are willing to sell one to you.