As low as possible, but not so low that your PC cant handle it. Set the "buffer length" first in foobar. Lower it slowly in increments, play a track after every step and see if the audio stays intact. Listen for jumps or pops, set it higher when this occurs. When you can multi-task and listen at the same time it should be about right. Do the same for the buffer size in the audio-gd control panel as well.
Here's a good example, that's on a 3 year old quad and playing from a standard hard disk, if the ASIO is working it should say "Active-Start" when playing something. As for the the Blue screen you might want to completely clear the driver off your system, restart and then install it again cleanly.
Edited by spamaerie - 9/25/12 at 9:08am