What is the purpose of your upgrade?
It looks like it is for gaming.
The graphics card is much more important for gaming as a rule than the cpu. And most games do not use more than two cores. I suggest you stick with the E5400 and upgrade the graphics card. The 7750 will be a nice boost, and the corsair 430w is a good and sufficient psu replacement.
The 430w unit will support a card as good as a 7790 or GTX660.
The FX-4100 is an ok cpu, but the need to replace the motherboard will require you to buy a new copy of windows since your current os is undoubtedly a OEM version which is tied to the original motherboard.
No game will use more than 2-3gb of ram by itself, so 4gb is ok. In a new build, I would go 8gb and a 64 bit os.
For gaming, I think I would just upgrade the graphics card to the strongest one your budget and psu will allow.
Leave the motherboard and cpu upgrade to later.
Another upgrade which I heartily recommend is a SSD for the os and some games. 120gb will do it.
If you get 240gb, you might be able to clone your current system to it and repurpose your hard drive for storage or backup. Samsung 840 and Intel 335 would be my first choices.
I am building it for Gaming. Not hardcore but enough to run Games BF3, Assassins Creed 3, Crysis, GTA IV at med-high settings or low but smoothly. I live in India so prices here are more than in the US, also if you can suggest any other components or PCs, it will be great
Those games, I think , are more graphics bound than cpu bound.
A great graphics upgrade would probably be best.
One exception I can think of is BF3 multiplayer which can be very cpu intensive, to the extent that even a FX-4100 may not be satisfying.
Look at this cpu gaming hierarchy chart for approximate gaming capabilities of various cpu's organized by performance tiers.
Not on this chart are the recently launched haswell cpu's. In general, they will be priced about the same as the ivy bridge predecessors, but with lower power requirements and a touch faster at stock. http://www.tomshardware.com/reviews/gaming-cpu-review-o...
BF3 single player is more graphics than cpu bound.
When you buy a pre assembled/built pc, you usually have little control over the quality of the components.
That can make a future upgrade difficult. For a cheap basic pc, pre built can be a good value. For a gamer, usually not.
My suggestion is to upgrade your graphics card to something several tiers better. Perhaps a 7750 or GTX550ti class card or better.
A good graphics card can be carried over whenever you think you need more cpu power.
BF3 should be more cpu-heavy than other games. It would make sense to upgrade it, especially since your cpu is way out-dated.
If you can't afford to upgrade your whole rig right now, just save the money.
Building your own is the cheapest way, and you get all the parts you'll need without some extras you don't.