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.