Okay this ani't about cpu but I have made a list on what you need to make certian versions of windows fly on hardware with a certian amount of ram I did not have time for cpu speed but the minum requirements are not what you want to use.
windows 3.11 and Ms-dos 6.22 16-24MB Ram
Windows 95 osr 2.5 32MB RAM use 64MB if you have the spare ram it is a minor speed increase
Windows 98SE 128MB Ram huge optimistation 96MB Ram little optimisation 64mb little as well 32mb optimsiation is a must 16mb don't even think about it
WindowsME 128mb ram more If you need sercirity software
Windows NT4 96MB 128 If you want to run lots of things at once.
Windows 2000 256MB
Windows XP 256MB Ram basic 512MB Advanced 1024MB Extream.

Win 3.1 & DOS work perfectly on 4 MB of RAM.
Win95 will work on 4, but 8 is a big leap in its speed.
Win98SE runs fast on 128 MB... BUT 256 is even better.

O okay but windows 98 only needs 128mb to be fast 256mb is for powerusers and they would run win2000 or xp.

Neh. 2K is notorious for its compatibility problems, and XP would turtle.
256 MB is good when you are doing a few things at once, for example running a P2P client (eMule in my case) and several browser windows.

Calculation is easy:

more memory = less pagefile use = faster performance

MS-DOS and Win311 / Wfw3.11 all work fine with 32MB/64MB.

Windows XP works best with 2GB of RAM, Windows Vista 2-3GB

AFAIK Win XP is not able to use use more than 1 GB of RAM.
At least so I was told so... :unsure:

Windows XP can use far more than 1GB

Oh, really?
My workmate told me that everything above 1GB RAM would slow XP down because of some sort of memory management error...
He read that in a IT-news page. Have you heard something like that? :huh:

XP has a hard limit of 4GB which is a hardware limitation as well as a software one. 32 bit processors simply cannot address more than 4GB (2^32 bits) and while I believe there are some Intel-specific extensions to deal with enlarging the address space, I double checked the Microsoft documentation of the issue and all 32b versions of Windows are still subject to this limit.

While the address size increases considerably with 64 bit processors (to the tune of 18 exabytes), I'm fairly certain that 64b Windows imposes a far lower software-level limit.