I just installed Windows 7 on an 8+ year old machine I had laying around my house and I was surprised to see that it runs as well as it does. In fact, it runs better than the XP installation I had installed on the old PC before upgrading it to Windows 7. To be fair, the Windows XP installation was a few years old and probably suffered from disk fragmentation and memory being consumed by the random applications and services that had been installed.
Hardware that I had difficulty with in Windows XP was automatically recognized and had the proper drivers installed. Even the cheap-o USB wireless stick I bought a few months back was recognized during the installation so I could setup my wireless network before even booting into the full operating system. On my windows XP installation, I had a bunch of problems getting the USB stick to work and had to get drivers directly from the manufacturer. With 7, everything was automatically recognized and installed.
Upon first boot into Windows 7, I noticed that my sound-card was disabled in the bottom right corner. I didn't worry about it at that point in time though and instead went straight to Windows Update. Sure enough, an update for my audio card was listed within the Windows Update panel.
As I mentioned, the computer is pretty old and still has the original onboard video card so I don't have the fancy Aero theme with translucency and some of the other cool features (gimmicks?) that come with Aero. There are a number of other small effects that are nice touches. It's nice to see the menus build with animations and they don't seem to impact the performance of the machine.
I'd really like to install this on a high-end machine to see how it performs, so I might have to pick up some new hardware to test it out. Specifically, I'd like to test out some of the new add-ons like Windows Virtual PC which requires hardware virtualization support. The OS is still in Release Candidate for the next few weeks and unless you are a techy I would recommend holding off until the full release. From what I gather, you will be able to purchase the OS in advance at a significant discount before it comes out.
Also, most of the manufacturers are now giving free or discounted upgrades to Windows 7 with computers that are purchased between now and the official release date. The RTM release should occur within July to give the manufacturers enough time to feed the OS through their processes to hit the October release date for PCs on our local big-box-electronics retailers shelves.
One more peice of advice: just because the operating system will run on nearly 10 year old hardware doesn't mean I recommend you running it on your old equipment. Go out and get yourself a new machine that will support the new features and sexy aero interface! You'll be happy you did so.