Doubt about RAM and PC Memory

[matskyblue]

Hello, good afternoon, my question is as follows: in January I plan to upgrade my PC, the new parts are these:
1- Intel Core I5 4570 3.2 processor (INTEL CORE I5 4570 3.20GHZ 1150 BOX :: APPInformatica.com ::)
2- Nox 650 W power supply (NOX POWER SUPPLY URANO VX 650W ATX :: APPInformatica.com ::)
3- NOX ATX COOLBAY SX case (for now, NOX SEMITORRE ATX COOLBAY SX CASE WITHOUT POWER SUPPLY BLACK :: APPInformatica.com ::)
4- ARCTIC REFR FREEZER 7 PRO cooler (ARCTIC REFR FREEZER 7 PRO R.2 AM2-3 775 1156 1366 :: APPInformatica.com ::)
5- Gigabyte Z87 motherboard (my two options are: GIGABYTE. Z87MD3H 1150 DDR3 32GB HDMI USB3 MATX :: APPInformatica.com :: or GIGABYTE MOTHERBOARD Z87 HD3 ATX LGA1150 :: APPInformatica.com ::)
6- Gigabyte GeForce GTX 660 OC 2GB GDDR5 graphics card (for now, if the GTX 760Ti is at a good price I will opt for that, Gigabyte GeForce GTX 660 OC 2GB GDDR5 GV-N660OC-2GD Graphics Card)

I will keep the reader, the hard drive (1TB, ) and the RAM (6 GB DDR3 1333MHZ, 2gb kingston, 2x2GB DIMM3 GU512303EP0202 2GB DDR3-1333 from the acer aspire AX3910)
My question is that I wanted to expand with 4gb more of DDR3 1333MHZ, since a friend told me that these RAMs should work well and I didn't want to spend a lot of money on higher frequency ones if there wasn't a huge difference, as long as they are good for gaming. What would you recommend?

[Obione]

Check prices at xtremmedia with the Hyper 212 EVO, a Seasonic S12II, and a Gigabyte 760 OC.