Yes you can. There is a Sonnet Aria Extreme N PCIe card available which is a straight swap for the Airport Extreme card in your Mac. It will give you N capabilities. But if you have a Core 2 Duo CPU in your machine, you can download an N enabler from Apple which will do the same (for $1.99).
Older equipment cannot operate wirelessly in the 5 GHz band. A firmware, or new card update will not supply the needed hardware.
'''Your only option:
Buy a USB 802.11n adapter' Whatever you buy, be sure that it works with the USB ports (USB 1.1 vs. USB 2.0) and has Mac software'' to support the hardware.
If you have a 2118 iMac it already has a -N adapter installed. What it didn't ship with was a -N driver. From memory there was some issue with Apple issuing a -N machine when they were built so they were advertised as -BG. When the -N drivers were released it was announced that this model and a couple notebooks had -N hardware installed and it was necessary to purchase the -N driver supposedly due to licensing. I bought the driver (under $5) and after installed the router identified the iMac as communicating using -N.