Need advice on buying Graphics card for my 2 year old PC.

My PC config is like this:

Processor : Pentium G620
Motherboard : Gigabyte GA-H61M-D2H Motherboard
Ram : Corsair 4 GB RAM
PSU : Corsair 430CX V2
HDD : 500 GB Seagate
Display : Viewsonic 19 inch widescreen 1440 x 900
OS : Windows 7 Ultimate 64-bit

Problem i am facing is 1080P HD video lags a lot while playing.

I am thinking to buy ASUS 5450 1GB card from snapdeal at 1755
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Please suggest budget graphics card in range of 1500 to 2500 or if my selection of Asus 5450 is fine. ?

Thanks.