There are a few bloggers wondering aloud about how well Windows 7 ultimately is going to run on older hardware. That's an interesting question, given Microsoft's symbiotic relationship with its hardware partners who constantly are in search of new ways to convince users to buy pricier PCs.
From early accounts by those using Windows 7 pre-beta and beta releases, Windows 7 is running pretty nicely on older machines with less powerful processors and less-than-optimal memory. In fact, as has been noted over the past few months, Windows 7 runs well on netbooks that haven't been able to accommodate Vista. (Microsoft has trimmed the size and memory footprint of Windows 7 while still building it on top of the Vista kernel.)
All that said, I'd bet Microsoft and its PC partners aren't very keen on seeing users run Windows 7 on older machines. They want to encourage them to buy new -- and preferably more expensive -- PCs. (That's a big part of the reason Microsoft and its OEMs are pushing multitouch as a new, hot feature for forthcoming Windows 7 PCs.) Even if Microsoft could support the older XDDM drivers on Windows 7, I doubt that it will do so, as that would send mixed messages about both its intention to move users to the new WDDM driver model, as well as its desire to get users to buy more new PCs.
It will be interesting to see whether Microsoft offers users the option to upgrade directly from XP to Windows 7 -- or whether it will only offer upgrades from Vista to Windows 7 -- once Windows 7 is released in final form. The question becomes even more pressing given that Windows XP will likely be a bigger competitor to Windows 7 than any other operating system out there (just like IE 6 is one of IE 7/IE 8's most formidable rivals, from a market share perspective).
If Windows 7 runs as well or better on your older PCs, will you consider installing it on your existing PCs rather than buying brand-new Windows 7 machines?