Welcome to Tom's Hardware, @Keverino!
Since you will be having a new OS onto the SSD, you won't need the Windows 7 on the HDD. When you get the SSD & Windows 10 installation, make sure you unplug the HDD from the system while installing the OS. Otherwise you are most probably going to encounter an OS Confusion with more than one SATA device connected to the motherboard. When installing Windows onto the SSD, make sure the SATA mode for that particular drive is set to AHCI and also check if it's listed properly in the boot order. After you have Windows 10 up and running onto the SSD, plug back your 1 TB HDD and transfer whatever games you are able to. However, keep in mind that it's highly likely that there would be some that won't be...