Kaushik 007 , you should always mention the reason or the tasks you want to perform when asking for a GRAPHICS SOLUTION in order to receive correct help...
If considering for gaming (I hope not) one is as vogues as the other.However if you consider one-to-one perfomance obviously the Nvidia card is a bit better with DirectX 10 gaming (shader 4.0 I guess)....not to mention the weak integrated G41 chipset.....And to answer your question the 8400gs is better than G41 chipset.
Note:a HD 5450 is dirt cheap and slowly getting out os the market this days.Its a lot better than the 2 you mentioned though...being a DirectX 11 entry level card.Let us know your purpose along with full system specs in order to get full help. Best of luck.