How to set the flash sync speed on a Nikon D40?

How so I set the flash sync speed on my Nikon D40 camera. What is the difference between sync speed and x-sync speed? I'm using a pair of Britek PS-300H studio light with a hot shoe adapter (AS-10) and sync cord. Should the built in flash be on TTL or Manual? What other settings would you recommend, to ensure that only the Britek studio flash is triggered to light the subject?

My goodness, you bought a bunch of stuff but didn't read the manuals.

1.x-xync and sync speed mean the same thing, sort of.x-sync is the default flash sync shutter speed. Usually 1/60th of second. Max sync is the maximum shutter speed you can use with any particular flash gear. Using that AS-10 and monobloc's, your max sync is probably no higher than 1/200th. It may be less. You will have to experiment, or read the manuals for the flash gear and camera, to find out.
2. That pop-up flash shouldn't be used at all. There's no reason for it to be up. If there were, it would have to be in manual mode, because TTL won't work properly with external lights.

You have to use this gear in manual mode. You have to match the flash power to the ISO and aperture you wish to use for whatever sort of shot you are taking. Shutter speed controls ambient light. If the flashes are the only light, you can use any shutter speed up to the max sync of your gear.