dude.
XP is based on 2000, which is based on NT, which works perfectly well with multi-CPU systems. Yes WIndows 7 will utilize quad core CPUs with HT better; XP has no issues with quad cores and dual cores and in all performance benchmarks with such products it comes out on top ;)
nope

Win98 on a Core i5 is also very quick, maybe even quicker as XP ? :p

Naah, think I get the point ;)

diminishing return actually ;)

but do try 98SE with Core i5... no drivers exist :p

Hmmm, interesting challenge :D
School's almost over, so it's time to format my computers... curious how 98SE is going to handle a C2D :-p