I would go for the GTX 560TI,have a look at the comparison here. But I do not think your current power supply will be enough you should have around 550W to be comfortable and you would need 2 6 pin power connectors.Specs. Here is the power specs for the GTX650 which need less power (400W) and only one 6 pin connector. Specs.
Microsoft Surface Pro 4 review: Hands-on with the new generation of Surface