Looking at your latest posting - a couple of things you mention at the end..........
"XP" which is not quick to load and taxes your CPU much more than earlier Windows versions.
"information, shopping, writing the odd letter or two"...... The internet is relatively the slowest thing on your computer and a faster box won't make much difference.
Letter writing again, no difference with faster computer.
But the way the retailing of computers is at present, you will save a lot of work and probably some money by carefully choosing a readymade box with its warranty etc.