So I was curious what the actual difference was in performance between XP and Vista. I decided to run a few test benchmarks to map the difference in performance. I ran 2 tests in each one with factory defaults for the Gpu @ 513/1584 then a bench with the card overclocked @ 672/2020. I left my E6600 @ 3501Mhz and the ram @ 830Mhz for all the tests.
First off 3dMark is a synthetic benchmarking program designed to test your setup for future games. Example 3dMark06 scores would relate to 2007 games and what estimated frame rates I should be able to pull with them. Say you score 10000 3dmarks that works out to 100.00 frames per second. 7123 3dmarks would be 71.23 frames per second. So you could expect that your card should run games at your bench settings at that framerate for games out in 2007. This is why you see a new 3dmark almost every year.
Now for the test results~ (Edited in 3dmark verify pages, CPU validate also added. Note I decided to validate that today last night I took my CPU to 3525Mhz but test results are at the 3501Mhz. I also validated with my computerforums username To lazy to go change it in the bios back to 3501Mhz though. Also for Vista you'll see the drivers are not approved why I don't know there on Nvidia's website version 158.24
CPU-z Validation Link
XP Home 32bit (9,385Marks = 93.85 FPS) @ Stock GPU (3dMark06 Verify
Vista Home Premium 64bit (8,930Marks = 89.30FPS) @ Stock GPU (3dMark06 Verify
Overall XP scored a total of 455 more marks than Vista got. Or 4.55FPS Not a huge difference for the same system settings.
Now the overclocked tests running the card @ 672/2020
XP Home 32bit (11,566Marks = 115.66FPS) @ 672/2020 GPU (3dMark06 Verify
Vista Home Premium 64bit (10,993Marks = 109.93FPS) @672/2020 GPU (3dMark06 Verify
Overall on the overclocked settings XP scored a total of 573 more marks on overclocked settings. Or a total of 5.73 FPS more than Vista.
It looks like you stand to lose roughly 5FPS on average making the switch from XP to Vista. Its hard to tell though when Nvidia comes out with new drivers they might close the gap further.