Problems with internet connection on my new iMac

I just bought a new iMac and for some reason I've been having a lot of problems with the internet connection going off several times a day. I was first using Safari and the connection was so bad that it would go out multiple times, very frequently, within a few minutes. I then downloaded Firefox and I seem to be having a better connection than with Safari; but nevertheless, the internet connection still crashes at least a few times each day. I have noticed that every time the internet goes out, if I restart the computer, the internet starts working just fine again. Although this seems to be the only way to "fix" the problem, it's not ideal having to restart my computer every 10-20 mins.

My iMac is placed directly in front of my wireless router (Linksys Wireless G 2.4 GHz 54Mbps) and is literally the closest computer out of all our computers to the router. The funny thing is, that the internet connection is working just fine for our other two computers (both Dells) in the same room. So this leads me to believe that there is something not working correctly (or not set up correctly) on my iMac.

Any recommendations? Any help is appreciated! Thank you.

Here are the specs for my iMac computer:

Intel Core i5 processor
6MB L3 cache and 2.5GHz processor speed.
4GB DDR3 memory
Multiformat DVD RW/CD-RW drive
21.5" display With 1920 x 1080 resolution
AMD Radeon HD 6750M graphics 512MB of GDDR5 video memory FireWire 800 port and 4 high-speed USB 2.0 ports
Built-in Bluetooth 2.1 + EDR wireless interface.
Built-in FaceTime HD camera
Built-in wireless LAN card (802.11n) Built-in 10/100/1000Base-T Gigabit Ethernet LAN with RJ-45 connector.
Mac OS X Lion operating system