The guy at eeeuser named kurttrail via GadgetMix conducted tests to find the difference on performance for both operating system Windows XP and Windows 7. Apparently, results tell Windows XP getting ahead on Windows 7 in all performance tests, as slated in the image below.
But Windows 7 is worth considering on upgrade since it has new pool of features ahead of XP. Plus the fact that Win 7 is said to be a much faster version of Vista, it is not that slow in for normal day-to-day usage.
You can head on GadgetMix for the full details of the benchmark.