4Gb of ram

do we need 4000 megs of ram for games or is it over rated?
im asking because im building a pc from scratch and have bought 2000megs already so..

im hoping to run windows vista, with firewall etc..

If you're not using a 64-bit OS and compatible CPU with mainboard, more than 2 GB of RAM won't do very much.
Peripheral component's RAM is mapped to addresses just below 4GB. Basically, this means your graphics card's RAM can be accessed by writing or reading to or from this "virtual" address.
When increasing physical RAM so it reaches into this address space, care has to be taken to remap existing devices into higher regions, or else physical RAM may not be addressed. As 32-bit computers only feature about 4 GB of address space, this can't be done--64-bit systems have a LOT more, so remapping can be enabled in the BIOS, paving the way for ludicrous amounts of RAM.
2GB RAM is sensible for a current 32-bit computer.