Ram upgrade okay for my computer?

Yes, it'll work fine. Yes, you can just add it to your current 4 GB. It will only run at 1066 MHz if you keep your current two as well, or 1366 MHz if you replace your current 4 GB, so you may as well get something a bit cheaper.
 


How about these? It has a promotion :).

http://www.newegg.ca/Product/Product.aspx?Item=N82E16820226095&nm_mc=EMC-IGNEFL020911CA&cm_mmc=EMC-IGNEFL020911CA-_-EMC-020911-Index-_-DesktopMemory-_-20226095-L08A

Also, would my system run better if i replaced the old ones then?
 
Slow down here, I'm almost 100% certain that you don't have 1600 MHz DDR3 running in your system now, so there's NO WAY to run the G.SKILLs at rated speed. More than likely, you have either 1066 MHz or just maybe 1333 MHz.
F3-12800CL9D-8GBRL http://gskill.com/products.php?index=239
AMD 790 Series
AMD 890 Series
CAS Latency 9-9-9-24-2N
1600 MHz

Use CPUz to identify your RAM -> http://www.cpuid.com/softwares/cpu-z.html ideally the two images are best otherwise post the info as follows.
RAM: please provide the following: Max Bandwidth, Part-Number, ALL JEDEC #4 information.
softwares-cpuz-05.jpg


MOBO: please provide the Chipset & Southbridge information:
softwares-cpuz-03.jpg
 



Slot 1 and 2 were empty but 3 and 4 weren't.

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