Thank you all, but my original qiestion (perhaps poorly worded) has more to do with the supposed advantages of the later processors.

For example, MMX technology has been available for about 10 years, and is included in all the Intel processors since then. I did some experimenting with it back then, but it all had to be done in assembly language. The results were pretty spectacular in the areas of video processing, etc. Sometimes a factor of 10:1 or more in performance.

However, if one doesn't use these instructions (and other later improvements), they might as well not be there. I'm presuming Microsoft and the other guys compile their code (C? C++?) and thus are dependent on the compiler to generate the proper instructions. And if they do, does the code notice that the processor has these neat instructions and use them, or do they just go for the fat part of the bell curve and get the stuff to market as quickly as possible?

Since the code seems to work properly on stuff as old as 486's, as well as a Pentium IV, I'm questioning whether I should worry about what "enhancements" a given chip has, since I might not ever use it anyhow.