260 is best of those 3. GTX 750 would be my minimum choice. Price is so close, the 260 isn't worth it. If you can afford to, 750ti would be a solid budget choice. I would recommend a cpu upgrade in the near future though.
GTX 750 will work in any PCI-E X16 slot just fine regardless if it is a 1.1, 2.0, or 3.0. Your pentium will handle it, but I would upgrade your CPU when you can. Crysis 3, the minimum recommended CPU, by THG, is a quad i5.