Hello Roy,
You are correct that the PCI-MIO-16E-1 can sample up to 1.25MHz. Since the Nyquist rate is only 625KHz, you might want to consider buying a card that has a faster sampling rate. Several of our S Series products offer speeds above 3 MHz. You can find more information on them here.
Alternatively, since the large signal bandwidth is 1MHz, you could alias your signal. For example if you had a signal at 700kHz, it would alias back down to 300kHz. There are some problems that can occur when doing this. For example, one would be if you have a signal that spans between 400-600khz when sampling at 1MHz. The better option would be to use a card that has a faster sampling rate, but you can give aliasing a try.
I hope this helps,
Jesse O.
Applications Engineering
National Instruments
Jesse O. | National Instruments R&D