I am considering getting Windows 7 to play CIV and would like some advice from anyone already using it. Does CIV work equally well on the 32 bit and 64 bit versions? If so, which is better to get? My cpu is a 2.4 GHz Intel Core 2 Duo. Would it be worthwhile to pay the extra and get the Pro version instead of the Home Premium one?
Civ V can run on XP pro, Vista (any version) and Win7 (any version) but for the best graphics you will need Directx 10 or 11 that XP cannot use.
Civ IV can run on any version of XP, Vista or Win7 but you may need to run in compatibility mode and download the latest version of DirectX 9 from Microsoft for vista or win7.
Win7 Professional is not required to run Civ IV or V but Home Premium is missing some nice features that come with the professional or ultimate versions which you may wish to consider outside of the requirements for the game.
I run Civ IV on a Win7 x64 system. The game does not recognise Win7's version number and therefore see's the system as below recommended specifications (I have seen people running Vista x64 having the same issue), the solution is to run the game in compatibility mode for Windows XP (service pack 3), which is available on Home Premium.
I am not convinced that you will need more than 4GB of Ram for the game to run successfully, so Win7 x86 would be fine for the game but you would need to consider what other processes you will be running at the same time, which would impact on memory usage.
x86 (32bit) systems can use no more than 4GB Ram so outside of Civ it is worth considering x64 (64bit) systems which come with many enhancements including being able to use up to 16GB's of ram,
there is no lower limit on Ram as such (although you would need some memory of course

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A bigger consideration than 8GB of Ram is graphics.
Integrated graphics chips use the system memory which has a big impact on performance where games are concerned and a dedicated Graphics card should be used.
For Civ IV I recommend a card with 1GB or more of its own memory, which is more important than 8GB of Ram to play big mods in my view.