To elaborate
@Ashishnag
but i dont have graphic card
all i have is a inbuilt graphic memory
can any driver or software can incrase the graphics??
The issue has to do with CPU, GPU, RAM, and other peripheral data that is simultaneously bussed. Think of a computer's bus as, collectively, wires that transfer data back and forth between the components. Depending on the system's hardware, only so much data can be transferred at any given moment.
One issue with integrated gfx is that it uses the system's RAM, unlike dedicated gfx cards which have their own RAM. Up to a point, as Wishmaster notes, integrated gfx suffices for basic functions, which would include low-end games, e.g., Myst I or Age of Empires. As the complexity of the game increases, more RAM is required by the game itself which, in the case of integrated gfx, is "stolen" from the system RAM in greater quantities. Eventually a point is reached whereby the system bus cannot handle the quantity of the traffic generated by the game AND the system: there simply is not enough wires.
AlacrityPC that Mpcrsc562 mentions attempts to attenuate system demands on the bus by shutting down non-essential services/processes. Simultaneously, demands on the RAM are decreased - all of which leaves more wires for the game.
If gaming is your goal, I would suggest that you not consider gfx cards in a vacuum, i.e., what are the capabilities (e.g., native resolution) of your monitor, power supply, how much RAM, etc. and buy/build accordingly. There are also pragmatic considerations such as the number and nature of expansion slots: will a gfx card (that may have an onboard fan) fit in the box among other cards? You get the point.
Finally, like CPU speed, number of cores, and amount of RAM, many embrace a bigger-is-better philosophy that just does not manifest itself in most computing. As a start in your decision-making process, you might consider this
article which addresses gfx card RAM issues.
Hope this helps.
Monk