I'm using PCI 6515 digital inputs to acquire a 64K quadrature encoder pulses attached to a motor with maximum speed of 3000 RPM. The encoder has two options of resolutions/revolution (32K or 64K). At this rate, with 3000 rpm speed - or 0.02 revolution/sec - the encoder produce pulses at 2.73 MHz (if 32K resolution is selected) or 5.46 MHz (if 64K resolution is selected). I looked in the datasheet and user manual of PCI 6515 (http://www.ni.com/pdf/manuals/372172b.pdf) but I couldn't find whether PCI 6515 digital inputs can successfully acquire or detect the encoder pulses at this rate.
Very doubtful, by my reading of the specifications. The section labeled data transfers does not include DMA, only the much slower interrupts and programmed I/O. The manual referenced change detection but nothing about a timing engine.
Even if you could capture the digital signals, you'd still have to decode it into position information in software. It's much more practical to use a device with a counter that has built-in support for quadrature encoders. It'll be able to handle those signal speeds, and it will do the decoding in hardware, giving you incremental position measurements directly.
NI's X-series multifunction boards would be a good place to start looking.