Hi LarryPutman,
The VXI -USB card uses USB 2.0 and will allow "32 MB/s sustained throughput across USB and VXI". VXI-MXI 2 has a max throughput of 38 MB/s (I could not find the throughput of VXI-MXI). If your older PC has USB 2.0 ports, I think you would be just fine using the USB card.
Regards,
Peter C.
Applications Engineer
National Instruments