So I'm doing a little research, to upgrade my old Pentium to a PIII or PIV or Athlon or whatever. From the descriptions, these chips are faster for two reasons:

1. Higher clock speed.
2. Improved architecture and extended instruction set.

I can see how the improved architecture can help things out, but what about the instruction set (MMX,SIMD, etc.)? It would seem to me that the MMX instructions, for instance, would certainly speed up things like graphic rendering and the like, but if the program doesn't use the instructions, they might as well not be there.

My copy of IE5, for instance, runs on the Pentium just like it does on my Pentium IV. Do they have conditional instructions so that they can take advantage of the enhanced instructions, or do they just go with the lowest common denominator?

So --- I'm trying to get past the hype. Why worry about some neat feature if you can't use it?