GTX 750 Ti 2GB

That particular GTX 750 Ti requires a 6 pin PCIe connector, which I doubt your tiny PSU has. You need to find a GTX 750 Ti that does not require an aux power source.
Normally, a 300W PSU is the minimum required size for the GTX 750 Ti. http://www.geforce.com/hardware/desktop-gpus/geforce-gtx-750-ti/specifications
But I am running this one in my Dell Optiplex SFF with a 240W PSU with no issues, so far: http://www.gigabyte.com/products/product-page.aspx?pid=5160#ov
If you decide on running a 60W GTX 750 Ti on your 240W PSU, you do so at your own risk... like I am.
 


I thought so too but check his card out: http://www.geforce.com/hardware/desktop-gpus/geforce-gtx-645-oem/specifications

It got 130W draw so I assume there is a 6-pin for it.
 


List the data on or take a pic of the PSU's nameplate.
I will say this in your favor... I have a Dell Optiplex 790 SFF with a tiny 240W Dell PSU (single 17amp +12v rail). My Gigabyte GTX 750 Ti has been running fine on it for quite awhile now. But Your brick might be different depending on its specs.