coolant leak

Hi Guys

I have a 97 Altima. I recently changed my radiator (the old one cracked). Its leakin coolant I dont from where. I checked the radiator it is dry from every where. It is coming from where the pessure bottle is. But the bottle is also good.
I have to top of the radiator every week it takes like 1 lit approx every time. and the level in the bottle remains the same all the time.

Do i need to change my radiator cap ? If, then what brand is the best ? Is it going to effect the coolant system ?

Right now i am using the old cap from the previous radiator.

YES, change the radiator cap. Go to advance auto parts and buy an OEM cap. However, this is probably not where you're leaking fluid, or else you would have seen coolant all over the top of the radiator (even so, REPLACE the CAP).
--The next spot to check is the outlet screw on the radiator... Make sure that its secured (its on the bottom of the radiator).
--The next spot you should check and probably replace if you see signs of wear and tear or excessive flexing is the radiator hoses. Check the top and bottom hoses and make sure that they are secure onto the radiator and engine, and replace them if necessary.
--In addition, make sure you're coolant overflow bottle is not leaking, and check the hose connection that connects the bottle to the top of the radiator.. Those are all the potential leak spots i can think of.
--Make sure there are no cracks or rips in any hoses.