Well...if you want and can afford a new system...just get it
There are few of us who buy new systems because they are unable to perform properly (IMO)...most of us buy a new one just because we want one with more current capabilities/specs.
If it's working without problems...that's the only test worth noting regarding functionality.
I've used PC Pitstop before (to see what sort of data it would spit out). In general, it won't reveal anything significant to you and it's geared to give generalized opinions based on the database it has collected (the site collects system data about every visitor who goes through the procedure).
A better guide, IMO, would be Event Viewer in XP, as well as some idea re what you might want the system to do that it cannot do today.