Hi Rushi,
In short, yes you can. So long as the device in question has a removable antenna, as a couple (a few Thompson and Netgears for instance) have fixed antennas that cannot be removed. The socket is usually standard so any after market one ought to work.
Although it's looks as if you're using a basic wireless G router, first up I'd replace that with one of N standard as this will give your network greater ranger and throughput potential (will always depend on the cards installed in the devices you connect). Second, you can try dual band. A standard router works in the 2.4Ghz region which gives great range but doesn't offer amazing bandwidth, 5Ghz however is of higher spectrum and thus offers far higher bandwidth in comparison.
Hope this helps,
Darren