96kHz across the board (unless you are specifically wanting to work in a DAW with samples @ other sample rates) in which case 44.1kHz might be more suitable depending on the hardware you are using.
Since Vista, Windows has this annoying niggle whereby a lot of soundcards/audio interfaces do not sync to the sample rate set in the sound preferences on the device. I am not enough of a programmer to be able to tell you how or why but at the end of the day even if your soundcard is set to 96kHz/24-bit you might find that windows is running all of its sounds at 44.1kHz/16-bit. This confused the life out of me when I first loaded up 7 as I found all my games/apps/general windows sounds to have a nasty amount of compression on them and I couldn't work out for the life of me why?!
In the end, I discovered that Microsoft rewrote the way Windows interacts with soundcards (something about off loading them from the kernel to try to prevent BSODs?) resulting in end users having to set their sample rates independently to match up their sample rates and bit depth or else face audio degradation. Don't just take my word for it have a look for yourself! Pull up your device driver software for the Xonar and select 96kHz. Now go to the sound preferences in the control panel of windows and right click on the default playback device. This will usually be called "speakers" if you are using stereo/2.1. Go to Properties -> Advanced and see if the sample rates match.
Also, remember that if you set your sample rate at 44.1kHz/16-bit you are effectively limiting yourself if you encounter any audio processing from an application/game that has been encoded at a higher rate. By setting a sample rate of 96kHz and a bit depth of 24-bit however, you will still run any material encoded at 'CD' quality (44.1/16bit) but have the added advantage of being 'HD' enabled, if you will, for other sources (not to mention the problem I described earlier with Winblows).