Just posted about this in another thread. Any add-in NIC card that has dedicated processing chip rather than using your PC's CPU will probably increase throughput. Realtek chips AFAIK use the system CPU to do the work, and so you'll be CPU limited in that scenario.
Intel's NIC cards are quite highly rated in a lot of places I've read, but as I said, to see a noticeable increase in performance (in order to justify the price outlay), get one with a dedicated processor. Even then, you'll still be limited by your other hardware (Router, switch - or lack thereof, cabling, etc.).
NAS1: Netgear ReadyNAS - Pro 4 [FW: 4.2.23 - Drives: 4x SpinPoint F4 2TB - RAM: 4GB]
NAS2: Netgear Repertoire U4 (Chirpa Special) [FW: 4.2.23 - Drives: 4x WD Red 2TB - RAM: 4GB]
Router: Netgear DGN3500_16M [FW: v1.1.00.33] | Switch: Netgear GS108Tv2 [FW: v126.96.36.199]
UPS: APC BackUPS ES-700uk [1st Gen]