Your first question should be;
"Do I need to do this, specifically for FFXIV? What else am I using that needs overclocking, and if so are they lagging?" If FFXIV:ARR is running at 40-60fps (outside crowded areas), then you don't need to do this, you're better avoiding it.
The difference will be so small overclocking a 650ti that it's pretty much not worth your bother with that specific card.
But if you're adamant to do so and are feeling like you just want to experiment, it's essentially a trial and error process where you slowly increase it bit by bit, if you see that it's stable you increase it more and you keep doing so until you see artifacts which looks like this;
Lines or square that aren't meant to be there after overclocking, IMMEDIATLEY start tuning it down until you find the highest stable clock you can find. Your GPU can also completley crap out and had a display driver reset/failure which can kick you to your desktop or lock up your PC forcing you to do a forced-reboot. After turning it down a bit, it's advisable to reboot, seeing as artifacting is usually remains until you reboot your computer (or something the 3D application in question)
THIS IS RISKY, you /can/ potentially fry your card doing this, and in general over-clocking some cards (as some are very very scalable) will lower it's overall life-time.
DO THIS AT YOUR OWN RISK.
Consider your power-supply, do you have the wattage to overclock your GPU?
Does your GPU idle at 40-60c already? (If carefully monitor your overclock as it'll increase the temperature).
As this guide suggests, increase it in small small increments, (it's tempting to just go "I'll do 50-100mhz, lolwutdotheyknow" but you're shooting yourself in the foot as it'll make it harder to find your GPU's overclocking 'sweet-spot'.
ALSO BEAR IN MIND, if you have a factory-overclocked GPU take extra caution as they've already tweaked it to a 'way below' the stable pont, so it's more likely to start giving you problems.
http://www.overclock.net/t/112/guide...cking-your-gpu
PLEASE PLEASE PLEASE read multiple guides online and try and look up your specific GPU and how others have gone about overclocking it.