hirajhil :
well i do not have multiple monitor just one 27 inch 1920x1080 monitor. I just want to have the settings around high if not full high. and what is eyefinity, i have heard of it. how can I be benefited from this if I have one monitor only.
Eyefinity is AMD's technology to run multiple monitors from one card. Since you have only one monitor, it won't benefit you at all. (It won't hurt you either.)
Neither the 7750 nor the 650 ti boost will give you high or even medium settings on modern games at full resolution. You can lower the resolution to another 16x9 ratio like 640×360, 854×480, 960×540, 1024×576, 1280×720, 1366×768, 1600×900 and still play full screen. If you resolution is small enough, you can get high settings, but it's a trade off of one quality for another that only you can answer.
It sounds like you might not be satisfied with either card. You should crack open the case, and see if you have an extra 6 or 8 pin connector on the power supply. Then considering your other components, determine the maximum wattage you could safely go up to for the card. Then pick a card under that wattage. (I did my best to calculate the theoretical maximum to spare from PSU and then subtracted 20% for age and safety.)
Picking the best graphics card for your power supply is a black art. Some will oversimplify it by using the card's TDP (not accurate) or the card vendor's recommendation (way conservative overkill). Read Tom's reviews for the card's maximum watts. To determine the maximum your PSU can spare, that's where the black art is. You'll need to ask others more experienced than me.