Honestly it doesn't sound like you are killing the computer most of the time, the most taxing thing is going to be converting the stored camera streams to dvd format, i do something similar on a old dual core, it just takes longer. Adobe can be a bit pigish at times, but I really don't see why you couldn't get away with a decent quad even with all that going on. You are not rendering. I also don't see the need for the openGL acceleration that the workstaton GPU's provide for that situation, unless you start using maya type products. Really any low end gamer GPU sounds like it would be more than enough, my 9800gtx is more than enough for photoshop and blender.
What i would focus on is reliability of everything, it already seems everyone is pushing you more to the workstation world so, maybe a nice enterprise class board, ecc ram, and esp. storage with 4 and possibly a 5th stream piped in what 10 hours a day you might want to consider back up options/RAID options/possibly enterprise class HDD's depending on how "mission critical" the data on them is.