Based on ZDNet lab tests, Vista SP1 provides equal and in some cases better performance than Windows XP SP2. It's always tough to benchmark different operating systems, as various settings may need to be adjusted so that there is equal footing for performance testing.  In earlier tests, there was a very slight advantage for XP SP2, so YMMV but not substantially enough to make that a point of differientiation in selecting which OS you may want to use.

Vista SP1 verses XP SP2 performance benchmarks
http://blogs.zdnet.com/Bott/?p=369

QUOTE: The first rule of benchmarking is: Your mileage may vary. On Ed Bott's test bed, with one exception, Vista SP1 was consistently as fast as or faster than XP SP2. Why the difference from Adrian Kingsley-Hughes' experience? Ed has a few theories.

Earlier tests
http://blogs.zdnet.com/hardware/?p=1332
http://blogs.zdnet.com/hardware/?p=1338