All systems that I have can run Vista. Actually, they all can run Vista 64 bit. My oldest computer (a Sempron 2600 with 1 GB of RAM) wouldn't have many resources left when Vista is loaded, though. XP Pro uses far less resources, so that's what I run on that Sempron system.

I run Vista Ultimate 64 on my laptop. Couldn't find some essential drivers for XP64, that's why.
I've found that several utility apps don't run well  on Vista 64, utillies like Spanki's STOMP and Objaction Scaler.
It's far easier to manage XP than to manage Vista, especially when it comes to networking. In XP, I can easily find out what's wrong (if there is something wrong, that is. Doesn't happen all that often), and I can easily correct it. With Vista, things aren't as easy to manage.
Security out of the box in Vista is better than in XP. But configuring Vista to make it usable is more work than configuring XP to make it safe - especially since I run an Active Directory domain so I only have to join the machine to the domain.

On my laptop, I've disabled as many of the gadgets as possible, and I use the classic Windows 2000 user interface. It's clean and far less resource hungry than Aero. Still, Vista 64 gobbles up 700 MB of RAM on its own, while a clean XP 64 installation uses less than 200 MB.

I'll skip Vista for my desktop systems. I'm curious as to what MS will put into Windows 7 - I expect that they will think long and hard about why Vista doesn't take off the way they thought it would, and I hope they'll make Windows 7 into an OS that is both usable and secure, leaving out the unnecessary gadgets.

There's a new trend in hardware too. Fanless computers. CPUs and GPUs designed to use a minimum amount of power, not designed for maximum performance. Fanless PSUs with modest power output (300W) are already available. It's possible to build a completely fanless computer nowadays, but it sure doesn't have the raw computing power that is required by Vista.
So if Microsoft is smart, they'll make a lightweight version of Windows 7 that'll run fine on these fanless PCs. Right now, XP is the only Windows version that will not overload these machines.
I expect that the fanless PC will have a sizalbe market share - low power consumption, absolutely silent, it'll be great for a home theater PC.
As for powerful PCs - I want to use the CPU/GPU/RAM for my applications! Not for the OS.

In my opinion, despite some good ideas in the system, Vista is a step in the wrong direction, and the market has rightfully slapped MS on the wrist.

The pen is mightier than the sword. But if you literally want to have some impact, use a typewriter

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