only thing your going to notice is that levels will load faster, it wont help with performance of the game. FPS is FPS, tied to your gpu and in some cases at lower resolutions your cpu. i keep windows on one drive my sammy 840 and games on the larger 850 evo, that way if the os drive gets borked i dont lose other stuff or if the gaming drive goes down i dont lose the os drive.
you have nothing to worry about get the new ssd and throw your games ect on there and later on when you can/if you want to replaced the os drive. though in reality even with higher bw ssd's in gaming or day to day things you are not going to notice any difference in performance or functions of the pc.
*edit*
i still have a second gen SSD, the OCZ vertex EX 120gb SLC. i use that in my HTPC, using a mobile cpu, boot up between it and the newer 840 on my desktop. i see no difference in boot up speed or anything else i do with my pc's as in gaming or watching videos ect. not take into account that the EX has much lower BW being on sataII, the SLC drive has the same 4k read speed as the 840/850. i seem to have lost my pics but i can rerun the tests to show what im talking about. now you can see my post in the review thread for the intel 750 i have posted some pics. the 4k write is what im talking about they are still below 50mb/s and in many cases between many different drives are not that far aparrt. now unless its a bad controller i havent seen in those cases where they are more then 10mb/s apart and thats a small difference. the vertex EX is still competive in that aspect but the biggest thing to keep in mind. is that access time, you go from a drive that is in the MS say 4-8ms in some cases to an ssd that is below 1ms and in the .00's. to borrow a pic from the 750 review
http://www.overclockers.com/wp-content/uploads/2016/06/graph-asssd-access.jpg
you can see how the drives are the access time be it sata or pcie based are about the same. being access time might be up to .02ms-.03ms difference in some cases is something your not going to notice vs a drive with 4ms-8ms. we are talking near instant access to the data on the drive.
the only thing faster then an SSD in bw and access time are RamDrives. those have fallen to the wayside with SSD's picking up pace and prices dropping so much. you can still find software that will let you make ram drives but that uses you system ram. with how cheap ram is and a board with enough ram you can get a little over 256gb setup. you could do a network attached drive that way but then your limited in bw to the network and its latencies.
