4GB of RAM in iMac but only 3gb being possible to use

sorry if the title makes no sense basically, in the iStats toolbar under memory it says that 1GB is always being used even if nothing is open, i am abit curious as to why that is...
I'm running 4 GB 800 MHz DDR2 SDRAM on a 2.4 GHz Intel Core 2 Duo

sorry if this is an obvious question!

Thanks

Well your Mac needs memory to run the OS and everything. Naturally that'd take up some processes too even if you just let it run without opening anything.