Software itself do not require power. The hardware they use, they do.
Typically a hardware configuration for word processing and spreadsheets can be very basic, you can take pretty much any PSU the market offers today.
For CAD though you might need something more powerful, but depends heavily on what are you going to do there. If you only use ACAD to print drawings you can also take basic hardware and stick with any cheap power supply. For heavy 3d modelling though you might need powerful hardware and thus much more power suppliance.
Of the three listed, Autocad is the one that should have the most resources. I say "should" because Autocad will run (on one of my laptops) with 2 GBs of RAM and a rather slow Pentium 4 processor. Regens are slower, but bearable.
I have also run Autocad on my i7 computer with 12 GBs of RAM; regens are faster. Circles and curves look like circles and curves as opposed to polygons. I also like to work with at least three of the most recent versions of the current Autocad design I am working on. Another thing is that 'Copy & Paste', 'Mirror', etc will work faster.
Based on my usage, I suggest a Core i7 computer with >6 GBs of RAM for Autocad. If you have Win 7 as the OS, buy the Pro or the Ultimate version that will support Virtual PC (XP Mode) for older XP based programs.
Any computer with a Pentium 4 (or better) and 2 GBs of RAM will handle Word Processing and Spreadsheets. PowerPoint too.