@rakesh the mobo you are considering is not a proper for your proccy and RAM, until and unless you OC it.
neither is ASUS P5N-MX suitable for the proccy and RAM
I suggest you consider good onboard graphics mobo based on nVidia 630i chipset like from
XFX 630 (price 3.5K),
Palit N73PV Motherboard(less than 3K), they would be enough for old playing games, saves your money also, so that if you really wanna go for any GPU u can get one whichever you think is good.
7150 onboard GPU is much better than intel x3100 onboard GPU.
In 8400 and 3450, I would say 3450 is better choice.