It's totally different case. Can't compare graphics card max memory support to 32 bit OS limitations. So no modern graphics are not 32b. As you see this issue is only with GTX970. There are graphics cards up to 12GB on the market ( didn't check exactly what is max but I believe 12GB was max for workstation gfx in mass sales ).
I'm not sure if all this memory issue is Nvidia marketing team fault or they were trying to hide something. I guess we won't know that for sure. I only know that marketing departments usually get only small part of info about architecture and they pass to the public whatever looks good ( often without checking it ). In this case 4GB looks better than "4GB but .... " what could cause that many potential clients wouldn't be sure if it's good idea or not and would pick competition. Actually there is no big competition at this price point.
For enthusiasts and overclockers it's a huge difference. For gamers counts only that it's not performing at full speed at 3.5GB+. For people in sales/marketing it doesn't matter if it's graphics card, a car, table or whatever as long as they can sell it. Most of them can't understand what we see in this specification.
Still 99% people buy computers like they buy a dishwashers or microwaves so it's an issue but in large scale it doesn't really matter. Nvidia is sorry about it ( more sorry they lose reputation ) but will move on and I doubt anyone will remember about it in couple of months. Except maybe those who made cases in court as they will wait 1-2 years.
My theory if it makes sense in anyway would be, its possible this was done on purpose by Nvidia, since they are way ahead of AMD as AMD has been on the slowdown in the CPU and GPU industry but because we're talking about GPUs here since they compete with Nvidia and are the only two in the market, we'll keep it GPU based. Its possible Nvidia could later on get fined for monopolizing the market, is one way of looking at this and price gouging since their GPUs jumped in price and that some GPUs who some of us believe should be their middle tier are their premium overpriced top tier.
Actually the biggest graphics card manufacturer is still Intel with about 60% market shares. It's because of office computers where companies replace hardware more often than usual home users. Still most home users are not playing games ( or don't require expensive graphics card ).
Nvidia can't monopolize market with not much higher % sales than AMD ( if not lower but I don't remember how it looks like recently ). Other thing is that they have different departments, technologies and slightly different target consumers. I mean all that graphics calculations and business market where AMD is barely counting.
If we look at the prices then AMD went after Nvidia and raised prices the same for each generation in last ~7 years. I remember when high end cards cost $300-400 on both sides and there was nothing above that. Now it's $600-800.
Just before GTX600 Nvidia raised prices and lowered specification just because they had problems with mass production and they saw no big competition. Almost year later they were so proud that they have the highest marge on the market. Regardless of specification etc consumers were buying all these graphics cards, Nvidia wasn't lowering prices and AMD had nothing better to show.
AMD is not much better. They were serving us the same graphics chips for 3 years but refreshed with not big differences, still under new series. So what they really did in last 3-4 years ? Improved drivers and additional software.
Simply on the IT market there is always lesser evil but it's never perfect.
