seriously though, you would have to be running many computer strenuous programs to notice...and thats if you put then 3200 and the 3000 side by side. For a normal computer user, 3000 is all you need.
The short answer: yes you can use the same copy w/ the same serial number. Microsoft doesn't seem to notice that two or more people have the same serial numbers.
The legal answer: No, most versions of XP pro are a (single user license)--only to be used once. There are versions sold that have multiple user licenses but, these are more expensive corporate/business copies that consist of anywhere between 5-500 user licenses.