Memory question for Noob

Hey guys i'm kinda a noob with computers so forgive if this is a stupid question. Is more than 4gb of ram necessary for a gaming rig? The reason i ask is because i usually though games need to have like major memory space to run properly. I have 4gb (2x2gb) DDR2 memory and i was playing Saints Row the Third and i thought about this question. While playing SRTT i minimized the game, opened firefox and a whole lot of tabs, opened Windows Media Player and put about 100 songs in a playlist and started playing the songs on shuffle. I also have Picasa opened with a loaded library of hundreds of photos. Now i opened up Windows Task Manager and the Resource Monitor and it says that i'm only using 2.2GB of my 4GB ram.

If all that takes only a little more than half of my 4GB why do people have 8GB or more of RAM in big gaming rigs?

BTW i have Windows Ultimate 64bit, AMD Athlon II x2 250 @ 3.51Ghz, J&W JW-A780LM mobo, 1TB Samsung HDD, Gainward GTS 450 GS(Golden Sample) 1GB and then the 2x2GB of DDR2 800 RAM