OC w/unmatched memory - 2 dimms (TWIN2X1024A-6400) & 2 dimms (TWIN2X2048-6400C4)

[wingzerotype]

Hello, when I built my system (Core 2 Duo E6600 Conroe 2.4GHz with an ASUS P5W DH Deluxe Mobo) about a year ago I built it with the following 2 dimms:

 

CORSAIR XMS2 1GB (2 x 512MB) DDR2 SDRAM 800 (PC2 6400)

TWIN2X1024A-6400

Timing 5-5-5-15

Voltage 1.9V

 

A little while back I saw a good deal on a good memory upgrade so I bought and installed the following 2 additional dimms:

 

CORSAIR XMS2 2GB (2 x 1GB) DDR2 SDRAM 800 (PC2 6400)

TWIN2X2048-6400C4

Timing 4-4-4-12

Voltage 2.1V

 

Now I want to OC my system, so armed by reading through online articles and this forum for a few hours I went into my BIOS and tried a few different things until my POST displayed that I was booting at 2.6GHz processor speed and PC2-6400 (I made sure to set my mem voltage to 2.1V). However, when I went and looked at my information in CPU-Z, it posted the following timings:

 

http://img519.imageshack.us/img519/7540/cpuz1yw4.jpg

 

I understand that it should run at the slower timing of 5-5-5-15, but why is it running even slower than that? Before I changed the Bus speed from 266 MHz to 289 MHz the BIOS was on an "auto" OC profile and it ran the mem at PC2 5300 I think, and the timings were the appropriate 5-5-5-15. Is 5-5-5-15 just incompatible with the higher bus rate or does this have to do with the fact that I have 2 different types of different memory dimms? I noticed most other people are using all matched dimms, so I'm not sure if thats causing any of my trouble. Thanks for any help, I'm new to this! :D:

[Xtreeme]

Just dont go higher volts, ram set one is low volt, ram set two is high. Not a good mix really. Only thing you can do is set the 5-5-5-15 manually and see if it sticks. If it doesnt the bios is messing with your timings.

[wingzerotype]

Should I lower the voltage or just forego the overclocking alltogether? If the bios is messing with the voltages does that mean its a bad setup to overclock? I'd like to overclock but I don't want to do it if its gonna risk burning out the low voltage RAM.