with dual processors you will need an OS that support it (NT, 2000, XP pro)
it does help with dual.
I used to have dual xeon 400MHz w/1mb cache with ultra160 10KRPM hd.
now upgrade my system to AMD 2000+ with same hd.
Both used to run on XP pro.
Boot up time, dual is alot quicker, once in gui I can start using right away, with single i wait about 5sec.
ms office apps launch faster on dual.
Install apps quicker dual.
but I do see performance increase in 3-d gaming with amd 2000+.
Note: higher cpu speed or dual does not constitute higher performance.
if you have pentium IV 2.4GH with ATA33 hd, doesn't do any better.
faster cpu + faster memory + faster hd (ultra 320) + faster video card = great performance but $$$$$$$