To answer the original question, it all depends on which games you'll be playing (also depends on the processing power of your computer.
Windows XP: Best for lower end systems & play older games (most DX9 games will run fine). The advantage of Windows XP is that the operating system barely uses processing power.
Windows Vista: I simply don't recommend, you should skip to Windows 7.
Windows 7: Works best for high-end systems. It allows you to access game libraries using features in all DirectX versions (up to DX11). The version you should go with (Windows 7 - 32 bit vs 64 bit) depends on how much RAM memory your system has. If you have 4 Gigs of RAM or more, I recommend going for the 64 bit edition.
PS: There was a project to bring DX10 to Win XP but the project has failed. So as it stands, there is no (probably will never be) a Windows XP with full DX10 feature capability.
Thanks everyone for helping and supporting me, now i have decided to install win 7 but my question is that which version of windows best for gaming 32,64 and 86 bit? and what is the difference between them? remember i have 4gb of ram.
In order to get the entire 4GB to be recognized and used by your system, you should use a 64-bit Windows. As far as I remember, if you're using a 32-bit operating system, it'll only show you have (and use) only 3.25GB. Actually x86 is an instruction set architectures based on Intel CPU, launched in 1978 as 16-bit extension of an older microprocessor. Since then, new, faster extensions have been created, x86-32 bit, and later on, x86-64 bit.
So my advice is, if you're going to use 4GB RAM or more, go with the x86-64 bit operating system. If you want to read more about x86, x86-32 bit, and x86-64 bit, click those links.
just get x64 as long as your cpu is capable
64 will run all 32bit programs and it will use all 4 gigs of ram and its good to get used to because your next computer will be a 64 bit os
the only reason to use a 32bit imo is if you have a 32bit processor
nope the reason to run 32 bit is if you have less than 3 gigs of ram the o.s will take up less space in memory due to it using 32 bit addressing, so the math takes roughly half the space knocking of 64bit a good third off the o.s footprint.
all games come with 32bit version so theres little chance of incompatibility and you will be able to use 16bit installers from very old games. something 64 bit cant do.
see there are some benefits to sunning 32bit. forget about xp also it is eol soon so no more patching and updates... win7 all the way... if you have more than 3 gig then 64 bit if not then 32bit.