Can I combine these two RAM sticks?

[Stg-Flame]

RAM 1: (Blue)
Mushkin 2x2GB
HP2-6400
5-4-4-12 1.8-2.1 V

RAM 2: (Black)
Mushkin 2x2GB
XP2-6400
4-4-4-12 2.0-2.1 V

Motherboard: ASUS P5K-Pro

I'm piecing together two older PCs into one-half decent PC for my girlfriend to use and I bought (seemingly) identical RAM for both PCs just a few days apart. All of the components were purchased from the same Newegg pages, but the RAM from Computer 2 is black whereas the RAM from Computer 1 is blue. I built these PCs back in 2009 so please keep the "why didn't you buy this instead..." comments out.

I'm going to test this PC with the new GPUs before adding the additional RAM as I would like to get some input before I start throwing in mismatched RAM.

 

[Elixer]

Can you? Sure.
Will it work, that is the big question... and the answer to that is, maybe.
The DIMMs would use the slowest timings & both have the same voltages as well.

So, you could try, and run memtest86+ overnight (at least) and see what happens in the morning.

 

[Stg-Flame]

Isn't there any other way to see if they are both working together? I've never tried Dr. Frankensteining a computer before and I've never used more than 2 sticks of RAM in any PC.

 

[Elixer]

Nope. That is the only way.

 

[fralexandr]

I've thrown ddr3 2x4gb 1333 with ddr3 2x4gb 1866 and run it at ddr3 1333 no problem.
If you don't care about ram speed, you can probably just use jedec/auto for the ram.

 

[Stg-Flame]

Well, I put them both in and the machine took forever to start up (thought something broke for a while), but I checked the DXDiag and it's reading 8GB instead of 4 and memtest came back good, so I guess it's ok. I just had some reservations about throwing mismatched RAM into a machine as I've heard horror stories in the past and this is the first time I've ever combined different PCs into one.

 

[Billb2]

When any computer starts up the BIOS reads the SPD tables (timings) programmed into the memory. It reads only the memory stick closest to the CPU. You can download and run CPU-Z to see the SPD tables in your memory. The BIOS then uses that memory speed and memory timings from the SPD tables that corresponds to the CPU speed that is set in the BIOS and then uses that speed and that one set of timings for all the sticks. So, you have the best chance of success if you put the worst performing memory in slot 1.