As every operating system has certain system requirements, what is the minimum specification required to install Windows 8.
There's probably 3 things you need to look at. Firstly there's a hard list of system requirements -
Any system that would run windows 7 would run windows 8 comfortably -
Secondly there's also additional requirements for certain features : multitouch for touch screen features, and a monitor of 1024x768 resolution for using Windows Store/Modern Applications, and 1366x768 to be able to 'snap' apps
Both these are off the windows 8 enterprise evaluation page
If you wanted a idea of what would * definitely* work, you might want to take a look at the systems that MS used to test windows 8 as well.
You can still use the old windows 8 consumer preview setup file to check compatibility on an older system running windows to be sure as well, until a release version comes out.
This should give you an overview of what exactly is likely to give you issues if you do update. As of October 2012, the setup will will check your hardware and software but not do an install, and is pretty small
I've run this on an old XP system from 2007 (which is apparently fully compatible with windows 8!) and it clearly tells me which applications and hardware would need fixing.
Microsoft states your computer will need the following specifications. Do note that Metro apps won't work on a screen resolution below 1,024x768 pixels.
Windows 8 Release Preview works great on the same hardware that powers Windows 7:
Additional requirements to use certain features: