Wow pentium D- that is old school!
I'd recommend getting an Intel 1150 socket motherboard and a Core i3 4XXX series chip. That should be fine for a 750ti and provided you get a good board you will be able to upgrade down the line to an i5 or i7 if you need to.
You will also need new ram (as the technology has moved on since you're machine). I'd recommend getting 8gb of DDR3 2133 memory as that should cover most things without costing too much.
I would look parts up for you but I'm in the UK so I can't get prices in $ I'm afraid.