Eric,
The hardware devices on the page you linked are much too slow for your signal at 840260 Hz. The maximum sampling rate for those devices is 102.4 kHz or 204.8 kHz, both of which are far below the Nyquist rate for your signal (> 2*840260 = 1.68+ MHz). Sampling near the minimum frequency which meets the Nyquist criterion will work in theory but may require a large number of samples to get good information.
Is the signal you want to measure on the 280 kHz signal or on the 260 Hz signal? Are you looking for phase or amplitude information? How fast does that information change?
Lynn