If you have a flat screen/TFT monitor, its frequency is 60hz, which means that they cannot display anything faster than 60fps anyway.
If your graphics card feeds more FPS than this to the monitor, its basically wasted energy IMO, but worse than that, it can lead to a degradation of image quality and produce a "tearing" effect on screen (eg. lines across the screen which make it look like its been torn or is splitting).
I don't think this tearing is noticeable in most racing games, but it can affect stuff like first person shooters quite badly.
Enabling vertical synch stops the graphics card from sending more than 60fps to the monitor, allowing it to spend more energy on pre-rendering which might help smooth out slight FPS drops (ie. drawing frames in advance of sending them to the display).
Vertical synch can be troublesome in some games or on some systems though, so maybe test it and see if it looks any better (or worse), and I suggest setting "max pre-rendered frames" to 3.
BTW: there is usually a setting for verticle synch in the graphics card options, and another one in rF and Race 07/Evo ... make sure you set them both to on or off just to be sure.
You should know if its working, because the max FPS you get will be 60.
BTW: I have this set and have a pretty solid 60FPS with all games.
A couple of other settings to look at that will help increase your image quality, reduce on screen "jaggies" and use your card to more effect than doing 300fps, are the AA and AF settings ... try shifting them up a notch or two and see if you can still maintain 60fps (or slightly more if you don't have vsynch enabled).
BTW: Vista is rather a bit hungrier than XP on system resources, hence the difference in your frame rates.