but mine is a 64 bit system and have core i 3 processor.
Yes, like I said: Using a 32-Bit program like a game in a 64-bit system and OS the game can address up to 4GB of RAM. Very few games run in 64-bit. So having 8GB available to your system will allow for your second 4GB to be used by things other than the game while still letting the game have as much as it needs. Anything that is not needed will act as a drive cache which helps with load times and speeding up application that you launch often like your web browser. That will help with things like making your system feel more responsive when launching things or loading data that you've previously accessed.
8GB is generally the sweet spot since most games run in 32-Bit Mode and can use up to 4GB of RAM (on Win7 64-bit) to themselves which leaves the other 4GB free for drive cache, other programs, and background processes.
but mine is a 64 bit system and have core i 3 processor.
Yes, like I said: Using a 32-Bit program like a game in a 64-bit system and OS the game can address up to 4GB of RAM. Very few games run in 64-bit. So having 8GB available to your system will allow for your second 4GB to be used by things other than the game while still letting the game have as much as it needs. Anything that is not needed will act as a drive cache which helps with load times and speeding up application that you launch often like your web browser. That will help with things like making your system feel more responsive when launching things or loading data that you've previously accessed.