Something like the HP NC360T which can be had in the US market for $20 or so in slot PCIE_16_2 would do the trick.
That's an older PCIe x4 card designed for servers, and it has 2 ports driven by an Intel chipset. Some other people on this board can recommend similar cards, perhaps ones that are available cheap in your region, but the main gist is that you can get an Intel server NIC with 2 or 4 ports pretty cheap on the used market.
I'd expect that you'd see significant benefit simply by using a second PCIe NIC rather than your current PCI one, no matter the chipset, but probably not realize 1Gbps throughput until you get something with an Intel chipset. The Realtek chipsets get a bad rap with FreeBSD and pfSense because the driver support is not on par with drivers for other operating systems, specifically Windows and Linux.
And finally, if your internet connection uses PPPoE for authentication, look specifically for a NIC that uses the 'em' driver for Intel chips. The HP I mentioned above does, as do the PRO/1000 NICs (as far as I'm aware). There is something about the 'em' driver that makes it perform better with PPPoE over the 'igb' driver that some of the newer Intel NICs use.