For "casual" gaming, and especially if you're talking about games like these, that are not particularly system demanding, there is no real need for an i5. COD has used the same gaming engine since COD2. the i3 is more than powerful enough. Actually, for games like COD and Assassins creed, you could probably get away with no discrete graphics card at all and go with an AMD APU (which has better onboard graphics than the i3) and just call the system complete.
Power supply is actually overpowered for the purpose. You could get away with a Corsair CX-430 and add a moderate graphics card later. Something along the lines of a GTX 750, 760 or a Radeon equivilent. I've got a home server/HTPC with a Phenom II 975, and a Nvidia 550 TI in it with the Corsair CX430 in it.. Even that graphics card thats a couple generations old could max out MW3 pretty easily.