i have an asus 2gb gt640 but it lags when i play games like cod bo is it due to my intex 450w smps gives only 18A current but my gpu requries 20A current or is this because of my 2 gb ddr 3 ram please help
A GT 640 is not really a "gaming" grade video card, it's not going to perform well in modern games, it's ideal for those who enjoy HD movies and videos.
As for the Power supply, don't worry so much about the Amps it puts out, it's all about the watts in most cases.
For example, a small PSU can handle your 640 with ease.