Hello,
Unfortunately, the NI USRP 2921 does not have the ablity to connect to a PC through the PCIe bus. The only way to connect your USRP to your computer is through gigabit ethernet.
If you are not oppposed to doing so, you may want to look into our NI USRP Reconfigurable I/O devices which do have the ability to connect to PCs through PCIe, PXIe, and ExpressCard.
Regards,
Cameron T
Applications Engineer
National Instruments