I'm currently running Windows 7 RC x64 on my main machine. The only problem I had was the driver for my wireless card seems to have been removed from this build. It existed in the beta, and it exists in Vista. No problem I thought, I'll just download the driver from the manufacturer's website instead, only to find that the drivers hosted there are unsigned. And the problem with 64-bit versions of Windows is that it mandates signed drivers. I attempted to grab the driver from my Vista partition but it doesn't work and gives an error, so the last resort was to run Windows in Test Mode to get the unsigned driver to work. Other than that hiccup, Windows 7 has worked flawlessly. Even the beta was stable as hell.
I also tested the beta on my laptop (that used a Pentium M Dothan at 1.86 GHz and had 1 GB of 400 MHz DDR2 RAM), and it just seemed to bring it back to life after its stint with Vista. Vista was by no means slow on the laptop, but it felt like it could perform a little faster at times. On the other hand, Windows 7 seems to fly on it.
Anyway from my experience, Windows 7 seems to perform a lot better than Vista on less powerful machines. On more powerful machines, the performance difference is a bit more negligible when compared to Vista. You will notice certain parts of the OS will feel a bit snappier though. I think gaming performance under Vista is rather similar to XP if you have the hardware to run it well. Windows 7 just seems to have much less overhead, but otherwise has similar performance. Overall, I think the biggest impact on gaming performance will be down to the drivers.
I've more or less been using Windows 7 as my primary OS since the beta. I still have it dual-booted with Vista, but I never really boot into it any more. I've had no issues with gaming on here. It's all been nice and smooth on my GTX 260.
Anyway I recommend that you give it a try, especially since it's free. Then you can get an idea of its performance and stability before the retail comes out.