At default settings you'll end up with a CPU bottleneck instead of a GPU bottleneck. The minimum CPU recommendation for Syndicate is an i5 2500s, quad core running at 2.5GHz. You have a dual core running just .5GHz faster at 3.0GHz. The minimum for Unity is an i5 2500k running at 3.3GHz, which has both a faster base clock and more cores to it. The games you want to play are open world games. Open world games are notorious for being CPU heavy games. If you had a newer pentium/i3 then you could get away with it--but at this point, playing it at default settings with your CPU, I would say don't expect more than 30FPS out of your CPU if you want a stable framerate.
The 730GT wouldn't work better than the 750Ti, it's a worse GPU, but it fits your CPU better. You have a bad CPU, so the logic there is to pair it with a bad GPU. Thus, the GT 730. If you're looking to save money, then I might consider getting the 730. But if you want to start playing games on your PC, you might want to start looking towards an entire system upgrade instead of just one component.