Earlier this year, ZDnet's Adrian Kingsley-Hughes got a chance to test and benchmark the latest beta of Windows 7 against Windows XP SP3 (32-Bit) and Windows Vista SP1 (32-Bit). He used 23 common tasks on two differently configured PCs to see which operating system performed better. The winner of most of the benchmarks is surprisingly Windows 7, which is still in early development. The article can be found below:
Windows 7 beta 1 performance - How does the OS compare to Vista and XP?: http://blogs.zdnet.com/hardware/?p=3236
As the exact benchmark numbers for the tests were not posted, this should be taken with a grain of salt. However, if true, the results are very encouraging for Windows 7, which most likely has not been completely optimized yet. Another interesting thing to note is that Vista slightly performed better than XP on the 4 GB PC, while XP and Vista traded blows on the 1 GB PC. This supports the fact that Vista can indeed perform faster than XP with more RAM, as Vista and Windows 7 are designed to utilize all RAM for program caching and quick launching, while XP only utilizes it when programs are actually launched.
As someone who was actually satisfied with Vista, I'm looking forward to Windows 7, as it is shaping up to be a performance tweaked Vista with several nifty features.