Hello,
Are you talking about FIFOs that buffer received data before it is transferred across the PCI bus? I believe what you will eventually care about are data transfer rates; it should allow for maximum GPIB transfer rates (whether using the high speed or standard protocol). The information on NI's website for this product is at (also see links to manuals, etc. at this page):
http://sine.ni.com/apps/we/nioc.vp?cid=1254〈=US
Best Regards,
JLS