What kind of hardware is necessary to run QGIS on Win XP?
In my experience, for common applications any hardware would do the job, for example, I have and old notebook which runs Windows XP and Qgis nicely on 1GB RAM and a Centrino (Pentium-M) processor.
As I always say, you won't need a lot of processing power to digitize vectors or interpret imagery, for that the best hardware to spend your money is a nice 21" led screen.
If you want to do some advanced processing with rasters (interpolations, etc.) you may want more memory and a better processor but be aware that most of these tasks don't take advantage of multicore cpus, so to have a real nice performance you will have to split the tasks and run many instances of whatever you want to process.