I switched over to i3's integrated graphics, and the performance gain was was clearly visible.
The WEI score for Desktop graphics Performance increased from 4.6 to 4.8 and that of Business and Gaming graphics from 5.2 to 5.6.
The score in MSI Kombustor also increased noticeably (I've lost the readings of GPU test) and frame rates in BF3, Driver San Francisco and NFS most wanted also increased by about 5 to 10 fps.
I guess I'll sell my 8400gs as it would increase my funds which I'm collecting for GTX 650.