I've built a half-dozen or so systems over the years,
but I've never really grokked video. Still don't.
My current goal: a Sandy Bridge system that can (ultimately) support 2 monitors.
I don't game (a character flaw perhaps attributable to my upbringing).
The most taxing use will be Lightroom & other photo editing.
I also record my tunes, so fewer fans is better.
I don't mind spending $100-$200 on a card or two -- but not if I don't need to.
It seems that high-end cards benefit one's gaming experience -- but what about photo editing?
1a) If I opt for a Z68 motherboard, do I even need a video card ...
1b) ... or perhaps, strictly speaking, I don't need it, but performance would suffer without it?
1c) in which case, should it better to get a P67-based motherboard?
2) If I need (or should get) a video card, is 512MB of RAM enough?
(Again, I don't game.)
3) For my purposes, any performance benefit to 128-bit over 64-bit?
4a) For a (future) second monitor, could one use the Sandy Bridge/Z68 video
(assuming that I've correctly understood what I've read), and then add
a modest (< $100) card for a future additional monitor?
4b) or do I need to find a board that will accommodate 2 video cards
(or a card that will accommodate 2 monitors)?
To answer all you questions, you will need a discrete, workstation card with dual monitor or more support on a P67 chipset motherboard. With your $100-$200, you will find entry- to mid-range workstation cards. From my knowledge of video cards and their VRAM, you generally need more at high resolutions, and considering you will be using 2 monitors in the future, you might need more. If you want the best there is to offer at your budget, you can get this:
By the way, you're pretty lucky that you don't game or have never gamed. Because of gaming, I got glasses and gained weight. I was the fat kid in 5th grade (I lost A LOT of body mass, but not much weight; now I'm skinny and nonathletic). With the money I blow off on gaming, I literally could buy a used car. I don't get put much now. Trust me, my life along with many others would be somewhat better without gaming.