What do you need to do with the device? The DAQmx driver will cover any of NI's relatively modern boards, including the PCI-6251, the PCIe-6251, and many many more.
I think most users would benefit from a newer X-series (PCIe-63xx) device. You might start by considering the 6351 or 6361 as shown in Table 1 on this page. Other options might be worth considering if you'd like to enhance capabilities in a particular area.
Odds are high that any code you have that runs on the 6251 will also run on any X-series board (as long as the new board has at least the same # I/O channels as the old). I know of one obscure possible exception related to buffered period measurement, but I seem to be one of the few who were adversely (and knowingly) affected. Otherwise, I consider the X-series to be nice improvement over the M-series MIO boards.
-Kevin P
CAUTION! New LabVIEW adopters -- it's too late for me, but you *can* save yourself. The new subscription policy for LabVIEW puts NI's hand in your wallet for the rest of your working life. Are you sure you're *that* dedicated to LabVIEW? (Summary of my reasons in this post, part of a voluminous thread of mostly complaints starting here).
