I'll be honest and say I have absolutely no idea what CAS latency, timings, or voltages on my RAM mean. All I know is that the RAM I was looking to buy was different from my current RAM in all these respects. I have 2x 1gb sticks on Patriot DDR2 800 PC2 6400. Newegg says the CAS latency is 4, the timings are 4-4-4-12, and it runs at 2.2V. The RAM I want to buy is G.SKILL brand, CAS 5, timings 5-5-5-15, 1.8-2.0V. Will these two different types work without any modifications? Also, if I were to try and modify settings would the settings have to be the same for both sets, or could I change them individually?

IIRC, you will have to retune timings to match the CL5 set of RAM. I would recommend you look for matching RAM, however, to stop one set from messing with the other.

As a general rule, mixing different types of memory like this is not advisable. I would suggest matching your existing memory exactly.

As a general rule, you shouldn't mix RAM that way. You can... I have... But it is usually less stable to do so. And you will end up losing the performance that you could have had with the faster RAM. Now, you could re-tune the RAM settings to where you are effectively overclocking the slower RAM, but if you were gonna overclock anyway, why not overclock the good RAM, and get even better results? I say it's alright to use some mismatched RAM to get you by for a while, but it shouldn't ever be any type of long-term solution.

If those are the ones you will choose though and they run at DDR2 800 already you may be able to put them up to 2.2volts to and get them to run 4-4-4-12. If those new modules run at DDR2 667 you will have to slow down your Patriots.

You can just avoid all this though and be full proof if you get one with like specs. I am mixing modules too. I have the same patriots as you and am mixing them with these. http://www.newegg.com/product/prod [...] 6820145015
Both are at DDR2 834 4-4-4-12 and 2.2volts stable.

In general, there shouldn't be a problem with mixing brands/speeds of standard DDR2 RAM, just like there shouldn't be a problem using a Purolator brand oil filter on a Ford car.
However, the two RAM types you listed are unfortunately NOT standard DDR2 RAM, as each needs to be overvolted (above the DDR2 standard 1.8V) to reach its advertised speed/timings. Worse, they each require a different overvolt:

All RAM installed in a machine needs to be run at the same voltage, speed, and latency settings -- they can't be set independently for each DIMM. Thus, without more information we have NO IDEA if there is a voltage where the two RAM models you mention can both run correctly.
Hopefully, both can run properly at the DDR2 standard 1.8V -- you need to contact each manufacturer and find out (a) will the specific module work at 1.8V and (b) what speed and latency settings should be used at 1.8V. These will of course be slower settings than the advertised overvolted settings.
Based on that info, you can pick a speed and latency settings that both modules will support at 1.8V. If the manufacturer won't tell you this info, they are being dishonest in selling their RAM as "DDR2" RAM.

I just bought a second set of RAM that I thought was compatible with my first one. My first set was this one: Patriot eXtreme Performance 2GB (2 x 1GB) 240-Pin DDR2 SDRAM DDR2 800 (PC2 6400)

The second set was GeIL 2GB (2 x 1GB) 240-Pin DDR2 SDRAM DDR2 800 (PC2 6400)

They are both PC6400, both with 4-4-4 timings, and can both operate at 2.2V, but I can't get my computer to boot. I had all the timings and voltages set to auto, but I get blue screens every time I boot. I tried again several times with new timings and voltages, but I still could not get it to boot.

The weird thing is that both sets of RAM work fine individually. When I use one or the other, they both finish several passes of memtest86 with no errors, and they boot into windows with no problems. There seems to be no problem with the RAM, and all the settings look the same, so I have no idea what the problem is. Any ideas?

What MOBO are you using and what are your complete system specifications?

It is possible that you have gotten 2 sets of memory that will not run together in the same MOBO.

You are using manufacturer-sanctioned overclock RAM at a fairly high voltage. It's often tough enough to get 4 DIMMs to work when using non-overclocked DIMMs -- try running them at DDR2-667 at 1.8V. If that doesn't work, boost the DIMM voltage by 0.1V per try.
This post may prove helpful: http://forumz.tomshardware.com/har [...] 81#1249881

ASUS P5N-E SLI motherboard, with an Intel E6600 processor. My processor is overclocked at 3500mhz, but the memory is unlinked so I'm not sure if that has any significant effect on the memory.

It does, you are also OCing the moemory when you OC the FSB. What memory divider are you using?

(waits for the "what???" response)

Actually, it depends on how the BIOS is set up. On Abit boards, in "unlinked" mode, changing the FSB does not change the memory bus speed.

Ok, my bad. I'm still stuck on s939, so I should stop making comments about newer stuff

Bah, tedfor just try and set the vDimm to 2.1-2.2V, that should make the RAM happy.

I don't mean to thread jack, but I have a similar question that I can hopefully find an answer for. I hope this may benefit you and I both. I have 2gb of Corsair XMS2pro ddr2 800 5-5-5-18 rated at 1.9v, and I just ordered XMS2pro ddr2 675 4-4-4-12 rated at 1.8v (pc-5400@675? interesting). I'm thinking that the voltages' being closer may make my set-up more stable than the one you proposed (both should run fine at 1.9v 5-5-5-18 ), but my 800 is way underclocked anyway -- 600mhz at 1:1 on a DS3+e4300 OCd to 2.7 [300x9]). When it gets here, I'll let you know what I do for stability, but it sounds like the difference in your sticks may require just a little more fine tuning. Does everyone think this sounds like a fair assumption? May I ask why you are not going for another 2 gigs of the Patriot?

sacto - similar voltages and same manufacturer/product line bodes well.

i didn't see the OS mentioned, are you running a 64bit OS? if not, then i was under the impression that anything over 2GB is nonaddressable and won't be utilized. is there something you are doing in particular that requires more than 2GB's? to me it seems silly to have more than 2GB's of Ram unless you are using some seriously RAM intensive applications.