My number one advice is the purchase the most card you can afford. This will help you to avoid buyer's remorse and wishing that you had just squeezed out a little more on your budget.
As you can see, however, its hard to say that the extra money will make that much of a difference in terms of playability or higher image quality. In general, the stock 750 Ti performs about 13% better than an overclocked 750. That sounds pretty good, but here is how it translates to actual games at a 1080 resolution:
Assassin's Creed IV: 3 FPS
Batman: Arkham: 5 FPS
BF3: 4 FPS
BF4: 4 FPS
Bioshock: 8 FPS
CoD: 9 FPS
Crysis 3: 2 FPS
Diablo III: 12 FPS
Far Cry 3: 2 FPS
Grid 2: 6 FPS
Metro LL: 4 FPS
Splinter Cell: 3 FPS
Tomb Raider: 2 FPS
WoW: 5 FPS
Besides the performance advantage, the 750 Ti has 2GB of VRAM memory, while the 750 only has 1 GB. While this may not have a major effect on your performance at normal resolutions, it
might come into play at some point. It really depends on what resolution you'll be using and whether you plan to play the latest most-demanding games.
If it were me, I would plan to eat some meals at home and use the extra money from eating out for the 750 Ti.
http://www.techpowerup.com/reviews/ASUS/GTX_750_OC/25.html