For MW2 and 3 a Core i3 is more than enough. I personally use a Core 2 Duo (i3 is basically what replaced it) and MW3 runs fine. Take into account that an i3 is faster than a Core2 duo.
And the GT 640 is also fine for MW2/MW3, I have a GT 440 and it, as mentioned before, runs MW3 fine. Also, again, take into account the GT 640 basically replaces the GT 440 and is faster than it. I get 30-60fps on max settings, 1080p in MW3.
BF3 is unplayable after the first level on my rig (so it might be the same for you). Black Ops runs weird on my computer (random fps drops), don't know why so might be same for you. Best of luck!
To Melv: I agree with your answer that yes a GT 640 isn't the best for gaming but for MW3 it's fine, same goes with an i3, not the best but it'll do. Considering he's only 13 and probably without a job I'd recommend saving money and going with the i3. Also to the asker, the GTX 650 is only 10-30 (depends) bucks more than the GT 640 and will be a much better card for the money.
You'll need a powersupply with a 6 pin PCIE power cord (I think the card may come with a molex to PCIE 6 pin power adapter but I'm not 100%) and depending on the one you get a case large enough to house a dual slot cooler. The GTX 650 is also quite a bit faster. GTX 650 Single Slot 1gb: http://www.newegg.com/Product/Product.as... Dual slot 1gb: http://www.newegg.com/Product/Product.as... Dual slot 2gb: http://www.newegg.com/Product/Product.as...
Save the money and go with the i3 as the graphics card is more important. A GT640 is a decent card and should handle the games you want at max settings but you should save up and get a GTX660 when you can as that will be able to handle more advanced games like Crysis at max settings and newer titles down the line.
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