You need to think more like a modern user and less like a programmer. Users generally don't care about memory issues, they just want good behaviors that work smoothly and efficiently.

The value of tabbed browsing has nothing to do with memory constraints. Sure, you can open a bunch of different windows, but then you the user have to manage them -- layering, positioning, making sure you know what all you have open. Tabbed browsing is like taking all the papers on your desk and putting them in a single pile (with tabs on each page so you can find the page you want, of course): suddenly you have all the rest of the desk to put *other* items on.

(Further, good tabbed mechanisms allow you to drag a tab out of the window into its own window, or from one window to another. So you don't have to have just one pile, you can have three or four, thematically grouped.)

And of course, there are user behaviors that aren't viable without tabbed browsing. Every morning on my desktop box, I like to visit a few news, blog, and webcomic websites -- 56 of them, currently. Hand managing 4 or 5 browser windows is one thing, but managing dozens of them isn't really doable.

Even on the N800, I like to open a small number (4-5) of similar sites and pop back and forth between them, dealing with a few items on each cycle. Being able to (a) open a raft of windows at once rather than one at a time, and (b) being able to click in the current window on an always-showing and same-location UI item to switch between rather than having to open the popup on the sidebar and scan to find the one I want and then choose it, those would be browsing boons.