Pairing RAM sticks

[Matt0087]

Hi. I have a MSI A88X-G45 GAMING motherboard. I currently have two RAM slots in use with two different size RAM sticks (one is 4GB, the other is 8GB); they were purchased at different times, but other than the size, they have identical specs and the same series of RAM (Viper 3). Is it a bad practice to mix two different sizes even if the specs are the same?
Here are the two RAM components:

• 
  https://pcpartpicker.com/product/rtPfrH/patriot-memory-pv38g160c0rd
  https://pcpartpicker.com/product/2B2kcf/patriot-memory-pv34g160c0rd

Now, I am thinking about upgrading to 16GB total RAM. Would it be okay to purchase a new Patriot PV38G160C0RD, which is identical to the currently installed part? or do I need to purchase a whole new pair of 8GB sticks?

 

[Tradesman1]

As long as they play nice it's not a problem, there are no guarantees when mixing DRAM...

 

[Matt0087]

As long as they play nice it's not a problem, there are no guarantees when mixing DRAM...

Is it really considered mixing if I'm buying the exact same part?

 

[Tradesman1]

Yep, the manufacturers test all sticks as they go to put sets together, you can pull sticks right off the assembly line and they might play, might not. Also the XMP programming is by the packaged set, i.e. the tRFC may well be much higher on a 4 stick set than on a 2 stick set, as well as the other secondary timings that you only see in the BIOS or SPD

 

[Matt0087]

Yep, the manufacturers test all sticks as they go to put sets together, you can pull sticks right off the assembly line and they might play, might not. Also the XMP programming is by the packaged set, i.e. the tRFC may well be much higher on a 4 stick set than on a 2 stick set, as well as the other secondary timings that you only see in the BIOS or SPD

It sounds like my best option is to completely replace the old RAM. So, should I buy 2X8GB or 4X4GB? Does one configuration offer an advantage over the other?

 

[Tradesman1]

2x8GB is less stress on the MC (memory controller) which is especially good for AMD as theey are designed towards a 2 stick setup

 

[Matt0087]

Which of these would be superior? They are all within $3 of one another on Newegg.

• 
  ■ G.SKILL Ripjaws X Series 16GB (2 x 8GB) 240-Pin DDR3 SDRAM DDR3 1600 (PC3 12800) Desktop Memory Model F3-1600C9D-16GXM (CL=9)
  ■ G.SKILL Ripjaws X Series 16GB (2 x 8GB) 240-Pin DDR3 SDRAM DDR3 2133 (PC3 17000) Desktop Memory Model F3-2133C10D-16GXM (CL=10)
  ■ G.SKILL Ripjaws X Series 16GB (2 x 8GB) 240-Pin DDR3 SDRAM DDR3 2400 (PC3 19200) Desktop Memory Model F3-2400C11D-16GXM (CL=11)

F3-1600C9D-16GXM (CL=9) has the most reviews and is a best seller, but it would have the slowest response in ns (based on this explanation: RAM Latency vs Frequency)

F3-2400C11D-16GXM (CL=11) would be the fastest. Should I go with that?

 

[Tradesman1]

I'd go the fastest - especially if using an iGPU. With those prices the fastest is a good move