I'm using Final Cut Pro and my friend and I are disagreeing on whether 2GB of video memory with the Dual AMD Firepro is enough or should I go with 3GB? If I go 2GB is the video memory upgradable? I haven't bought the Mac Pro yet.
The minimum amount of VRAM Apple recommends for running Final Cut Pro X is 256 MB. As a Final Cut Pro user myself (albeit a very light user), I find that 1 GB is sufficient, though I rarely put the program "through its paces," and the work you will be doing will likely be on a larger scale. 1 GB also happens to be Apple's minimum recommendation for editing 4K video and 3D titles.
Ultimately, it depends on how intensive the work you're going to be doing is. Are you going to be doing intensive work with 4K content? Or are you going to do smaller home video-style projects? It may be useful to use Apple's recommendations as bases for comparison as you make your decision (see below).
To view the entire list of minimum system requirements, see Apple's Final Cut Pro X Tech Specs page. This Apple Support Communities discussion also has some interesting details about VRAM and how it applies to Final Cut Pro X.
While gaming related, this post looks relevant to you. The long and the short is that VRAM actually affects video performance fairly minimally. The major gain is the ability to run multiple monitors/higher resolutions.
That said, the answer to "is enough" in computer circles is almost always "get the most" if you're planning on using a single monitor and editing 1080p video, the standard 2GB FirePros will be absolutely fine (Final Cut runs well on my 2011 MBP with a slower Radeon card and 1GB VRAM). If you plan on doing higher-end things (4k video editing at resolution with multiple clips and editors on multiple monitors), you will feel the upgrade.
Bear in mind all the advice you'll be getting will be relevant now, if this is a work machine that you can play on using for several (5+ years) and have the extra money, having more VRAM certainly won't hurt anything...