From the way you phrased your question, I assume you mean what's the difference between dual processor and single processor setups performance-wise, not specifically the difference between the AthlonXP and AthlonMP (btw, single-processor solutions are not refered to as XP, keep that in mind). The answer would be, it depends on what software you use. Games won't benefit much, if at all while multithreaded software such as Photoshop or 3D CAD would benefit, anywhere from 20-80% performance boost. And in more..........eccentric coding such as RC-5 or Seti, it could nearly double your speed.
"We are Microsoft, resistance is futile." - Bill Gates, 2015.