Mixing RAM

I think it works this way:

Your motherboard has 2 RAM channels, and 4 memory banks. The memory banks are labeled with alternate colors, something like Black, Blue, Black, Blue or Yellow, Orange, Yellow, Orange. Now the first pair of Black and Blue/Yellow and Orange is one channel. If you put the 4-4-4-15 sticks on the Black banks and the 6-6-6-18 sticks on the Blue banks, then they will run at their respective settings, assuming you didn't manually set the BIOS RAM settings. Otherwise, everything will run at the speed of the slowest RAM module. In this case, everything would run at 6-6-6-18.

Thanks for your reply, graceful. I would be putting the matched pairs in the same coloured sockets and I understand what you're saying about the slower latency being used if they are mixed rather than pairs in the same coloured sockets. What I'm not sure about is if they will run at their given settings if they are placed as pairs in the matched coloured sockets, are you sure that that is the case? Will different voltages be given to each pair of sockets, automatically if required? Or is it the case that the 4 sockets work as a whole in some way and that all four should have the same latency / voltage?

Preferably, everything should be identical. If not perfectly identical, at least they should have the same voltages. There is only one variable in the BIOS that controls DRAM voltage, so everything has to be 1.8 V or whatever. Although RAM modules can usually tolerate up to 2.0 V.

The timings will most likely be set to the slowest one, 6-6-6-18. If I were you, though, I would just get another pair of 4-4-4-15 OCZ modules. Do you need 12 GB that much? 4x2GB will give you 8 GB, more than enough for common tasks. Do you edit video or do 3D modeling?

Video editting and 3d, gracefully. I need the extra RAM so I can get some sleep! That said, I am tempted to go for the ocz rather than take the risk, I've seen people having a lot of problems mixing RAM this way but I'm trying to find out what the pitfalls are and if they can be overcome before I make a decision. If everything ran at the slower latency I guess it would still be a big overall increase with the extra 4gb, on the other hand I get the impression, from what I've read elsewhere, that things can go horribly wrong, when mixing latency / brands.

The ocz should run at 2.1 volts and the geil is stated as "Working Voltage: 800C5 / C6-1.8V/800C4- 2.0V". Do you know what those figures mean?

It looks like those specs indicate that the Geil RAM can run at CAS 5 & 6 with 1.8v but needs 2.0v to run at CAS 4 speeds. Mixing RAM of different brands, timings, and voltage is generally a bad idea. The best case scenario is that they will run at the lowest settings, worst case is that they won't work together at all. Which way it goes it up to the luck lottery. It's not a lottery I'm willing to play.  

Thanks Shortstuff_mt that makes a lot of sense re the figures. So if the geil runs at CAS 4 at 2.0v and my existing ocz needs to run at 2.2v ( I said 2.1v earlier but having checked it's 2.2v) what would happen to the geil at 2.2v?
I checked out the crucial site and they recommend, as compatible, an 8gb kit, 2x4, with a cl of 5 at 1.8v as being compatible.

I don't know what will happen if you run the Geil at 2.2v. All bets are off when running RAM outside their rated limits. You would be better off underclocking them and loosening the timings to see where they can do at 2.0v. Like I said it may or may not work. I wouldn't recommend running the Geil at 2.2v, though.

Personally I've seen computers with mixed sets of stuff all the time, they all work fine. Granted it is the whole they all run at the slowest speed thing. And most likely, you've never seen the faster timings that need more voltage unless you did it yourself in the BIOS. If you didn't it in all likelihood just ran with the 1.8 volts setting. Which is probably closer to what the other RAM is anyway.

The computer I'm typing this on right now has 2x512MB crucial ballistix tracer sticks and 2x2GB OCZ RAM. It's always worked fine. Before I had a secondary computer, it had 2x1GB Corsair dominator RAM instead of the tracer sticks. The corsair is now the only RAM in that computer. Oh, and an even better example is my server. It has 2x256MB RAM at the next slowest to DDR 400 (don't remember what it is) and then 2 different brands of 512 MB DDR 400 RAM and it all runs just fine.

As such, more than likely if you just shoved all of that in, it would all run at 1.8v with whatever the timings might be at that point. (which is what most people end up doing because they would look at you funny if you said the word "BIOS" let alone the rest of it)

Sounds like False_Dmitry_II has won the luck lottery so far. The problem happens when the motherboard tries to determine what speed and timings to set the RAM to by default. It just looks at the values of one stick and sets the values according to the specs on that stick. Those values may or may not work with the rest of the RAM in the system.

Interesting stuff, False_Dimitry_II, thanks for your comments which have given me cause for hope here. I've never set the voltage or timing for my RAM in BIOS. I guess I just left it on auto. Is there a way of checking the voltage while the machine is running or should I check and change things in BIOS and then do a memory test at the 2.2v to see if it makes a difference? If I know that the ocz will run OK at the lower voltage I guess the guile should be OK with it, at 2v.
shortstuff_mt when you say that the mobo sets the default timing do you think it would just go for the standard 1.8v or detect the higher voltage that the ocz RAM requires?

For the record mine is all running at 1.8v even though the ocz wants like 2.1 or 2.2 and the other wants like 2.0 or 2.1. My motherboard just doesn't let me mess with that unless I also want to mess with the voltage for the CPU, which I don't. So I just left it there.

Run CPUz. It should tell you what the different modes your RAM can run at and what one it's using now.

Thanks False_Dimitry_II I just ran CPUz and it looks like the RAM is running, automatically, at 2.2v. I see 4 tables along the bottom with different latency for different voltages and the spd ext is shown as being the one for the 2.2v. There are 4 options all together but only two for 400mhz. The other one is at 1.8v with similar cl to the geil RAM so I guess that as the geil and the ocz will run at 1.8v all should be well if I run them together at 1.8v and that I'll just get whichever latency settings are slower for both. If I'm understanding this correctly?

Yup. And it should get there without interference. Just post back the results.