GPU - Nvidia GTX660ti click here
PSU - Corsair Builder Series CX 430w click here
RAM - G-Skill 8GB DDR3 @ 2133mhz click here
CPU - i5 4570 click here
1.You seem to have a prebuilt PC ,please let forum members know the make & model with further details so as to confirm what hardwares it'll support.
2.600W PSU should be used to run a GTX660ti although 450W-500W can suffice .
3.<<(I've heard 1600mhz would be much better for RTS)>> I think you have 1333 Mhz DDR3 ram at the moment so you may not see much improvement(if any), & there is a great chance that your existing motherboard won't support 2133 Mhz ram.
4.i5 4570 is a socket 1150 Haswell CPU while your PC is a socket 1155 sandyBridge so it won't fit.