If you're experiencing latency (time-lag between playing a note on a keyboard and the sound being heard) make sure you have the ASIO driver selected.
Go to
DEVICES PANEL » DEVICE SET UP Select your audio-card ASIO driver ...then go to the Control Panel and lower the latency settings until happy.
quick tip:
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Press CTRL+ALT+DELETE » Processes Find cubasesx.exe Right click and set PRIORITY to MAX
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[taken from a cubase sx tutorial at:
http://www.thewhippinpost.co.uk/mixing_tutorial/cubase-sx-tutorial.htm].
about the sample rate: i'd use 44100, but if you have ASIO you may also use 48000 or even 96000.