CX600 will run any graphics card you want to install.
GTX750ti is probably appropriate to your cpu.
It is two tiers higher on toms hierarchy list than the R7-360. Not really a big difference.
On ram, 4gb is probably ok for games if you avoid multitasking while gaming.
Use task manager to monitor hard fault rates. If you see more than 1 per second, you probably need more ram.
Assuming your os is 64 bit capable, look for a 8gb kit of compatible ram. Speed is irrelevant.
Just adding 4gb may work, or not. Even the exact same pert number as your current may not be compatible.
Likely, yes, guaranteed, no.
You take your chances there.