Can I mismatch 5-5-5-15 with 5-5-5-18?

[DVL]

I have two sets of RAM, 2x1GB corsair balistix @1066 5-5-5-15 and 2x2GB corsair xms2 @800mhz 5-5-5-18.

I have a few questions regarding the RAM. Obviously the timings are different, one being 5-5-5-15, the other 5-5-5-18. What would be optimal to set them at in the BIOS? 15, or 18? Can I run them at different timings?

Secondly, the balistix RAM is set to run at 1066, never been able to get it there, sadly (any advice on that would be greatly appreciated) but the omz2 says 800 on the RAM chip itself. I've been getting a lot of errors BSOD, freezes, trying to get it stable, does anyone think it's because I'm o/c'ing the 800 xms2? I got them to 950. Is it because the timings are different.

Any idea's on how to get them stable?

[baditude_df]

Ballistix is Crucial RAM, not Corsair.
But generally speaking if you are combining RAM, as long as they're compatible with your motherboard and you are keeping one matched pair in one channel and the other matched pair in the other channel you should be ok. Timings and frequency would all be set at once to the same specs, and you would likely have to set the faster ram timings and frequency to meet the lower spec'd set to give yourself the best chance. It's not really advisable, but you can try it.
As for your stability issues, I can't help you. Sometimes RAM just won't run at it's specified settings. But likely you will need to give your NB voltage a couple notch boost to help out with your RAM, if stock voltage, freq and timings are not running properly.

[JackNSally]

Run them at the highest common denominator.
Going by stock speeds/timings would be 800MHz@5-5-5-18.
You should start then and then try bumping up the speed or tightening the timings. Start with one then go with the other.

[DVL]

Ballistix is Crucial RAM, not Corsair.
But generally speaking if you are combining RAM, as long as they're compatible with your motherboard and you are keeping one matched pair in one channel and the other matched pair in the other channel you should be ok. Timings and frequency would all be set at once to the same specs, and you would likely have to set the faster ram timings and frequency to meet the lower spec'd set to give yourself the best chance. It's not really advisable, but you can try it.
As for your stability issues, I can't help you. Sometimes RAM just won't run at it's specified settings. But likely you will need to give your NB voltage a couple notch boost to help out with your RAM, if stock voltage, freq and timings are not running properly.

Thanks for the reply, I was attributing my problem to the wrong component. I think my problem came from a slight o/c on my CPU, dropped it down .25GHZ and things are stable now.

I'd like to get the RAM o/c to the most possible. What would be the best method to get the RAM to the 1066 it's supposed to pull? Could the 800MHZ set handle it?

If I can get the balistix running at 1066 with 4GB's, would that be better than a slower speed, but 6 GB's?

[ou_phidelt]

Thanks for the reply, I was attributing my problem to the wrong component. I think my problem came from a slight o/c on my CPU, dropped it down .25GHZ and things are stable now.

I'd like to get the RAM o/c to the most possible. What would be the best method to get the RAM to the 1066 it's supposed to pull? Could the 800MHZ set handle it?
Go slow by adding small increments and voltage, test, rinse and repeat.
If I can get the balistix running at 1066 with 4GB's, would that be better than a slower speed, but 6 GB's?
That just depends on what you do wiith your system. For what I do I like lots of ram and tight timings. Others will benefit more from higher speed. Generally speaking most people see more improvement with larger amounts of memory than a small increase in speed. Particularly when you factor in Vista to the equation.