Skip the GTX 750 Ti, its slow for its price. Get the AMD Radeon R7 265 instead as a baseline, or R9 270X if you can afford it.
This buyer's guide is a great resource to help you find the best video card for your budget: http://www.tomshardware.com/reviews/gaming-graphics-card-review,3107.html
Your processor is very old and cannot be upgraded practically without also getting a new motherboard and RAM. It will probably be a problem in some games, but unless you plan on buying almost an entire new system there's not much you can do about it.