The ones I came home with were better than what I had before, an old Dell Inspiron 600m that is fast enough under Windows XP to do anything I want, but I do budget time with it. Since I hadn't moved into either of the two new machines I had the luxury of deciding "What do I want to do with all this stuff". Since I am an IT Manager now, I can actually put these things to use. The widescreen Gateway with the Core Solo processor which is still faster than the Inspiron is now my video editing PC and will get all the things that I do that take time. Set it up and let it roll on XP. It is stable and has a 17 inch wide screen, perfect for my web development under Joomla! (yes its a real name), and Video Editing.
I have this machine that was the fastest of the lot, an Acer Aspire 5610 with 2GB of memory. It is a 2007 vintage from what I can tell, not new, but in physically good condition and pretty much perfect for the upgrade. I have a (legal) copy of Windows 7 Professional that I won't share so don't ask. I started the upgrade then went for a dog walk. Yes, that easy. I didn't care what was on the machine and just formatted the extra two partitions after merging them and went out with Lettie The Super Dog for a mile around town.
When we got back, about 30 minutes later the Acer was sitting at a prompt asking me for a Key which I have. I sent it back on its way after a few more prompts, happily installing and set about my normal morning routine. It finished while I was in the shower and then I restarted it at the Out Of the Box experience screen - or OOBE (Ooo Bee). Yes, that is what we call it where you enter in your name for a log in.
Basically in short what I'm saying is that if you want a machine that is clean and you don't want a lot of junk running slowing you down, you will want to do a "Clean Format and Install". Go buy yourself a "Thumb Drive" of about 16 GB or so, it will cost less than $40, a copy of Windows 7 Pro (Skip the Home stuff, they leave too much out), and install it yourself.
If you have Vista, you can do an upgrade but I decided I did not want to go that way with it. I didn't care what was on the machine. If you do, the Upgrade procedure is not completely clean all the time, you may end up formatting the PC and starting over, so make sure you copy your My Documents tree onto that Thumb Drive (USB Key, Flash Drive or what ever name you wish to call it) so you don't lose your recipes and letters to Mom and pictures of the Dog. Also make sure you have copies of the programs you installed, back up your favorites and bookmarks to the drive, and just have a good long snoop around your PC and make sure you've saved everything you really DO need before installing.
If you have XP, back everything up because there is no way to do this without formatting your hard drive. The Install of Win 7 will do it for you, but everything on the hard drive will be gone.
Let me repeat... If you are installing Windows 7 on a machine with XP, you will lose everything so back it up first!
I have used Windows 7 since the betas for about 6 or so months, maybe longer. I've used it on some really strange hardware. The biggest question is what would you install it on. If your machine came with Vista, you should be safe - they typically come with 2 or 3 GB of memory and Win 7 is really happy there.
If you have an older machine, I would say that you probably have XP. Windows 7 runs just a teeny bit slower than XP. If you REALLY want Windows 7 then you can gauge for yourself but I'd recommend going to 2 gigs of memory first.
I have run it on a Pentium 3 1GHz laptop (IBM Thinkpad A30) with 1gb of Memory and it was useable but slow, however it was slow on XP. I wouldn't recommend it for long term use.
My personal opinion is that anything Pentium 4 or newer (Core Solo, Pentium M, Faster Celeron, Core Duo and so forth) with 2GB of memory will run fine. You really want a faster machine. Something approximately faster than a Pentium 4 2GHz but it will run on a slower chip and if you have a Pentium 4 "class" machine you're due for a newer PC anyway.
I agree with what Microsoft says the following at this link
If you want to run Windows 7 on your PC, here's what it takes:
1 gigahertz (GHz) or faster 32-bit (x86) or 64-bit (x64) processor
1 gigabyte (GB) RAM (32-bit) or 2 GB RAM (64-bit)
16 GB available hard disk space (32-bit) or 20 GB (64-bit)
DirectX 9 graphics device with WDDM 1.0 or higher driver