I'm sure a PC could handle 4 PCI cards if you have that many open slots. I could not find any info on an NI-PCI 7813R other than one other link in the discussion forums. Is this still a valid NI part?
I would recommend you go with compact Fieldpoint. You said you have the input points distributed over a big factory. Rather than runnin 500 pairs of wires from all over the place to a single PC, it would be better to network compact Fieldpoint network modules together. Scatter the network modules around strategically so that you can concentrate your inputs to specific locations and only have to worry about a handful of network wires to run to your host PC. Modules such as the cFP-DI304 offer 32 inputs. Youd could put up to 8 of those on a single 8 slot backplane. Then you only need two backplanes.
But depending on the layout of the factory, it may make more sense to use several backplanes and only put as many modules as needed for the particular location where the backplane will be.