Hi Stuart,
I would tentatively (not knowing anything else about your setup or workflow) say that yes, you should see a performance difference when running those two apps (plus others perhaps) on an 8 core, 12GB machine. Especially if you're using larger files. Not every function uses so many cores but just the general setup and compute resources there should provide some improvement.
Some added benefits include a much more powerful GPU for those functions now and in the future that will offload their processing to the graphics cards, tremendous expandability (4 drives inside the machine potentially, including dedicated scratch drive which can be very important), and the ability to leave other business apps (like email or browser) open in the background without worrying it will slow your machine down. Every Mac Pro I've purchased has been a great investment. Usually lasts a good 4 years and can go longer depending on workflow setup.
Would recommend you buy an NEC or Eizo CG-series screen for color critical work, rather than Apple's Cinema displays or cheap 3rd party display. Be aware though that if you buy a 10-bit screen, you will not yet be able to take full advantage of that on OS X. Hopefully that will come with the next big system update. There are more details on this topic in the Color Mgmt. forum.
The iMac would be a good option too, but for the fact that it's not nearly as expandable in terms of RAM, HD space, etc and the glossy screen. Last I checked Apple did not have a matte screen option with the iMacs (unfortunately). For me anyway, that would be a deal-breaker, but I'm one of these crazy people that likes having a tank of a computer under my desk. Keeps the office warm in winter!
Hope this helps some.