Hmm... for whatever reason my 2nd reply did not even post... I'll try and paraphrase what I typed before. Hopefully it will actually post.
There would be a large difference between 720p and 1080p. The GT 750m is pretty close to the performance of a desktop GTX 550 Ti and the Radeon HD 7750. Both would have about 67% the performance of my desktop Radeon HD 5850.
GTA 4 was not a very good console port. With my HD 5850 at 1920x1200 (just slightly more than 1080p) and medium settings my frame rates are between 22 FPS - 35 FPS from what I can remember. Of course that is with an older C2Q Q9450 CPU from 2008. If I had a more current CPU the performance will probably be higher at around 27 FPS to 44 FPs, but that is only a guess.
Rockstar has not announced when GTA 5 for PC will be released. I think GTA 4 for PC was released around 1 year after the console versions were released. So maybe GTA 5 will be out in Sept 2014.
You have the option to simply get the 1080p screen and play the games at 1366x768. However, this does lower the sharpness of graphics a little bit. You can test this out on your current desktop. Simply lower the resolution and play a game to see what the difference is. Just keep in mind that the laptop's screen is smaller than your monitor so the loss in sharpness will not look as bad.
I have a 14" Lenovo Y470 with a 1366x768 resolution screen. I find that it is okay for playing games. But when actually use the laptop for reading, surfing the web, or doing some work I find the resolution too low. I multitask and I like to see a lot of "things" on my screen. My next laptop will have either a 1600x900 or 1920x1080 (most likely) resolution screen. If I want to play games on it, then I will just drop the resolution to 1366x768.