nukemaster :
If I trust the TechPowerUp GPU database and Nvidia's own website the 750ti(stock model) is about 2 times more power hungry than the GT 610 you already have.
The real important thing here is the actual amount of of power you can get from your 12 volt rail(s) on your power supply.
Nvidia recommends a power supply based on a fully(fairly high end cpu and other parts) loaded system. It also takes into consideration the number of lower quality units on the market. You can have a 300 watt power supply with 192 watts on its 12 volt rail or a 300 watt with 288 watts(this is a pretty top end 300 watt unit) on its 12 volt rail(s). This makes a big difference since most of the system uses this rail now. Older systems used the 3.3 and 5 volt rails more, but as parts required more power, the moved to 12 volt was required to keep wire sizes in check. You need thicker wire to move 100 watts @ 3.3 volts(~30.3 amps) than you do at 12 volts(~8.3 amps).
With that said, you have what Intel specs as a cpu with an average load consumption around 80 watts. I say average because entire families of cpus get the same rating, but some are lower and some are higher(these numbers are at full load, power savings reduce this when the system is under lighter loads). Your video card is rated by Nvidia as 60(a pci-e slot is good for 75). Because your system was build with lower power in mind, chances are the board uses little power and a single hard drive(3.5 inch) can be from 5-10 watts under load.
If you happen to have access to a power meter that plugs into the wall, it would be a good way to check your systems current power draw. Remember very few system make it to 100% load on all parts(but some workloads can cause this). The 750 ti should not be much over 30 watts more(a bit more at the wall.).
Unfortunately I have no way of testing it as I'm on holiday, I have a wall meter and a meter that plugs in, in the back of the PSU and shows how much power is drawn and the average power drawn but they're both at my work place. You're correct, the board only uses one 3.5 inch hardrive, it also has no dics drives ect as I don't need any and I only plug one in when I need to insert windows disc for repair or format. Both fans inside the PC are powered externally by a fan controller which is power from a benchmark PSU. My system is fairly low power as it only powers the motherboard, CPU, hadrive and GPU.
From looking at GT 610 stats and 750 Ti stats, 610 uses 29w and 750 ti 60w, which is only extra 31w and with the 280w PSU I'm adding extra 45w, so not only I'm adding more power for the 750 ti, but infact, I'm adding extra power.
Obviously all these calculations are done on paper and all power consumption stats are written so in the perfect world, this would work, but since all of these are written, it will be different in real life. Dell intended for the Optiplex 960 SFF to be a small server PC to hold databases and I know they were never made to become a gaming rig, maybe thats why there is not much information out there and the whole fuss about custom board connectors that don't even have a name with a stupid exhaust cover design that prevents any card to fit except low profile GT 610 by Gigabyte without removing the cover and exposing board and southbridge to even more heat which is why I already bought a fan controller to indeed removal the cover so I can fit the 750 ti.
The Dell Optiplex 960 SFF was designed as a small server PC on budget that anyone could afford for their office to handle a database for 24/7 for whole year, due to that, it was made to be as low as power consumption and efficient as possible which is where the 235w PSU comes in as manufacturers try to put the smallest PSU they can. After doing that maths, everything points in the positive direction.
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