Users with modern systems should be able to upgrade without needing more RAM or processor power, as noted below:

Windows 8 – No increased hardware requirements from version 7,2817,2386339,00.asp

QUOTE:  Microsoft says Windows 7 users won’t need bigger and better hardware to run the freshly unveiled Windows 8. The PC system requirements will be the same if not less than what’s needed to run Windows 7, although the new operating system definitely appears tailored for the touch-screen interfaces of tablets.  Speaking at a press event at Computex in Taiwan yesterday, Microsoft’s Michael Angiulo said, “Windows 8 will be able to run on a wide range of machines because it will have the same system requirements or lower” as Windows 7. Microsoft Windows President Steven Sinofsky reiterated the approach at the All Things D conference in California.