Congratulations wowmix. I hope you will enjoy your purchase, and that it does what you need without any trouble.
As to sample rate. You should go to your Audio MIDI Setup application, which you'll find in the Utility Folder. Before you open it, close iTunes.
Open Audio MIDI Setup, and check that it sees your DacMagic - if not, then select Output Device, and you'll probably see the device, if you have it connected.
Then set 24bit as your wordlength, and 88.2 or 96kHz as your sample rate. If most of your music files are in Apple Lossless, then these will be encoded in 16bit/44.1kHz, but upsampling to 88.2 (doubling the rate), will be OK, and you can compare the two. Most people find that upsampling delivers a smoothing of digital sound that is quite pleasant. BeoLab 5s upsample to 24/96kHz as default, to provide another example.
Your DacMagic should recognize the sample rate chosen.
However, and this is important, do remember to either close iTunes before you set the rates in the panel, or else to close and reopen iTunes afterwards. If not, iTunes/QuickTime will operate to your previous setting, and not to the new one. The Audio MIDI Setup panel controls the output format of the audio from Macs - and has to be set manually, unfortunately, at least until Snow Leopard comes this fall.
Good luck, and don't hesitate to fire away if you have any other questions. I will recommend that you rip your music to Apple Lossless or at least 320kbps AAC in the future!
Cheers!