This book is pretty good - including especially helping decide whether some upgrade is worth it - The Complete Idiot's Guide to Upgrading and Repairing PCs, Fifth Edition
Author: Fulton, Jennifer.

Upgrading doesn't have to mean upgrading everything - sticking extra memory or a CD-writer in a machine, for example, can be very cost effective. For example I spend £30 on extrra memory recently, and now my PC crashes and freezes up a lot less often - and you can't get a new PC for £30, so far anyway.