On the 7344 the fastest rate at which you can do this is 100 hz. The method you should use is the high speed capture. Basically a hardware trigger that, when activated, stores the position where it happened into a registry which you can later retrieve. Our high end 7350 series controllers can do this into a buffer at a rate of up to 3Khz. You can find an example of using high speed capture here:
http://sine.ni.com/apps/we/niepd_web_display.DISPLAY_EPD4?p_guid=C04F7F32E01F0274E034080020E74861&p_node=DZ52480&p_submitted=N&p_rank=&p_answer=&p_source=External