Current games can be smoothly played at 1080p @ high to ultra settings on single GT755M but may be in future you have to lower down your settings or resolution to be able to play smoothly and 1080p definitely looks great on 1080p screen but 720p is no far behind. If you really pay attention to detail then only you will find out that 720p isn't as sharp as 1080p on 1080p screen.
If you are confused between HD and FHD screens than I would suggest go with FHD because you are not only playing games on your PC but watching videos, movies etc as well!
This is bullshit!!
SSD is like a HDD with same dimensions and same SATA & Power ports so HDD can be easily upgraded to SSD. GT755M also supports SLI and GT755M Ultrabay for Y510P is already available in some countries!
The gt755m can reach high at most and cannot render high end games like company of heroes 2, crysis 3 and ghosts in ultra.
I tried playing BF4 today in 768p in a fhd panel, the visuals looked pretty good to me.
The new Y510Ps have m.2 ngff(I think) SSD slots, just shown by an owner a few posts back.
Besides you can always remove the hdd and swap it for a 2.8" SSD.
The Ultrabay Gpu is currently only avaliable in Singapore. US models are coming with 755m SLI by default.
Now whether that GPU will work or not is not confirmed.
Some people successfully mix and matched gpus of the y series of lenovo, ie between. Y500 650m, y500 750m, y400 650m and y400 750m. They tried a gt750m Ultrabay Gpu in Y510P and found that Ultrabay feature was locked in Y510Ps which came with a single Gpu. Lenovo can probably fix this with a bios update if they decide to launch the second Gpu worldwide. Till then you can buy and pray and hope lenovo doesn't do what they did to me ( I bought the y500 to run in SLI mode which they never released in India.)