Gizmondo posted benchmarks of Windows 7 Release Candidate 1 and Vista

The first series of tests are the "everyday" ones—stuff you'd run into sorta daily, where taking less time is more better. Vista punked 7 out twice here, shutting down a bit faster applying a filter to a 16.6MB (9764x3720 pixel) photo in Adobe Photoshop CS4 with more haste. When we played Left 4 Dead using the recommend settings, Vista also seemed to deliver ever-so-slightly better performance, more consistently keeping the frame rate near 30 frames per second, though you'd have to be watching the FPS numbers rise and fall to really tell the difference between the two in gameplay. But you should keep in mind Nvidia's Windows 7 drivers are still in beta, and haven't been updated since March, while fresh Vista drivers came out a couple weeks ago, and drivers make huge differences with gaming performance.

Windows 7 Release Candidate 1 vs. Vista: First Benchmarks