The USB-6251 can only sample at that rate on one channel. If you use multiple channels the maximum aggregate rate is 1 MS/s. You should install an anti-aliasing filter before the input to the device. The filter bandwidth could be as low as 625 kHz ( the Nyquist frequency) or somewhat higher depending on the noise or interference which accompanies your signal.
The NI 9234 has a built in anti-aliasing filter. Since your samplerate is just slightly below the maximum sampling rate, you should not need any other filter. If you have an extreme case of a strong interfering signal just outside the signal bandwidth, a special filter might be useful
Lynn
