There was a time that Windows ran on multiple architectures. We had the PowerPC, Alpha, and MIPS CPUs in addition to the Intel x86. That ended with Windows NT as it seemed those other architectures didn't generate a lot of sales and Microsoft discontinued the porting of the OS.
For a long time we had different editions of Windows and SQL Server, and it was a fairly simple matrix to determine what level of CPU and memory was supported in each edition. However that's getting confusing again. It was hard enough in Windows 2000/2003 with SQL Server on x86 and Itanium processors to figure out the maximums, but now that we have both Itanium and x64 platforms in addition to x86, and different limits as Windows 2008 grows beyond the limitations of the 32 bit platform, it's becoming confusing again.
I noticed this last week with a simple question. How many CPUs does W2K3 Datacenter Edition support on the 64 bit platform? There was a good argument over the answer of 64. That's because we have Itanium and x64 limits being different.
You'd think this would be simple to determine, but it's not. And as Microsoft continues to shorten product cycles, we'll likely start to have 3 or 4 versions of Windows and SQL Server in many of our environments, making it more and more confusing for everyone.