Everything I have read (and implemented) on mobile designs has a max-width of 480px. And as you pointed out, people will just zoom if necessary. However, screen width is not the first thing to consider/worry about in mobile designs. Two more important issues is compatibility with as many mobile platforms as possible (a real nightmare) and being considerate to your users by limiting data transfer to what they need and not include superfluous information. You have to think about download speed and bandwidth usage (they may be paying for bandwidth usage). The other important consideration is making the link areas large enough so people with large fingers can easily use a touch screen. I41px high seems to be a comfortable setting.

But back to your OP, making it dynamic up to 750px, you might as well not even bother making a special mobile version, IMHO.

Marshall

The answer is neither; you want a responsive design. For example, a traditional 3-column site would have three columns of a certain size for fixed width, and three columns of varying sizes for fluid width, but the truth of the matter is that on a phone, you may have only space for one long vertical column in portrait and two columns in landscape, whereas on a 16:9 desktop monitor, four or five columns may make sense.

Just as no one maintains separate text-only websites, or IE- and Netscape-specific websites as we did in the early days of the web, having a separate mobile version of the size is increasingly a losing proposition. You have phones with 3-inch to 5-inch screens, 7-inch and 10-inch tablets, and all manner of desktop sizes, and unlike the desktop world, people are increasingly accessing your site from a multitude of devices and displays. Better to learn a responsive framework like Less, Golden Grid, Skeleton, or what have you now and build for the future.

Let me challenge everything Choster wrote above. Mobile sites are still necessary even if you use a responsive design. For the reasons mentioned above by Marshall, you have to limit the number of elements downloaded by mobile users even if you offer them a responsive design. Responsive designs are all the rage now, but they are not a perfect solution. You can through a responsive design limit some elements, but chances are that you'll still download huge js files and the likes to make your responsive site work.

On many projects I've worked on, we kept the client's mobile site because it was just that optimized and still a better experience for users.

There are other problems with responsive sites, even if they currently are the darlings of the Web development community. I like to be contrarian but test it out for yourself. Resonsive is not a one size fit all.