Keep in mind that PCI will *never* fully saturate Gb LAN. The PCI bus can't move data at the required speeds. ;)
Don't be silly. Going all the way back to the advent of PCI in 1992, where the bus speed was 33 MHz at 32 bits, you get a potential throughput of 133MB/sec, which is almost adequate to fully saturate gigE unidirectionally. In practice there's a fair bit of overhead but it is far more than the "Realtek" ("Badjunk" IMO) chip can cram out.
More current implementations support 66 MHz at 64 bit, for 533MB/sec, and PCI-X extends that into variants supporting up to about 4GB/sec. I've got several quad gigE PCI-X cards that can definitely saturate all the ports. The PCI-X stuff is mostly available on server boards though, and of course you have to sort through the quagmire of what your card and the PCI bus supports to see what your limit is.
Still, for a NAS, there is no reason that a home user shouldn't be able to get a satisfactory result from a quality PCI ethernet card like the Intel PWLA's.
Now if you want to take a shot at Realtek, be my guest. To get saturation of gigE, you need a high quality, no compromises ethernet card. That's true whether we're talking PCI, PCI-X, or PCIe.