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your power supply is going to dictate your possible upgrade.
I would go with a radeon r7 265 if your powersupply can handle it.
If you are going to upgrade, get the very best card you possibly can, don't try to save a couple bucks, spend to your absolute budget limits as far as your power supply will let you go. moving from a x1650 to any of the modern cards is going to offer a huge performance upgrade, may as well do it right.
Since the 750ti and 265 are nearly equal performance wise and the 750ti sips power I'll agree with the consensus here and say 750ti is your best bet.
I'd also suggest upgrading your cpu as well a Q6600 or Q8300 or something would be a good bet. I'm not sure about AC4 but BF3/BF4 will take advantage of the extra cores. If you don;t want to spend $30-$40 on a cpu upgrade you can prolly snag a E8400 for $15-$20.