Ram voltage

It is generally advisable not to mix RAM if possible. I would suggest purchasing the same type of RAM.
That being said, 1.8v is the standard for DDR2 RAM. If you are not overclocking, you shouldn't have any issues, and you can just run both at 1.8v. If you are overclocking, you will have to run both at the same voltage, so be wary.

thanks for the info cal. i 've got a small overclock on my q6600 go from stock 2.4 to 3 ghz. got fsb at 333x9 multiplier. i've got 2x1gig of corsair at the moment at 1.9v if i put 2 more sticks in of corsair 1.8v will i need to reduce ram voltage to 1.8 or will overvolting the new sticks from 1.8 to 1.9 to match the older sticks be ok?
i've also read that if you populate all 4 ram slots on the p5k it may well bsod when booting so wondered what 3 sticks do as far as the duel channel and single channel config question goes. totaly new to overclocking and voltages so thanks for the great advice.
cheers

if you buy 1.8v sticks instead of 1.9v sticks they might work, but then again you will void your warranty.

as suggested above, it is always best to not mix and match ram as you "are limited to the performance and specifications of your slowest/least performing module".

i'm not familiar with the p5k, but if you want dual channel you have to add ram in multiples of two. for example on a 4 ram slot board a few scenarios i will list below. named items in each row indicates a slot filled by a ram module:

SLOT 1 + SLOT 3 = dual channel
SLOT 1 + SLOT 3, SLOT 2 + SLOT 4 = dual channel
SLOT 1 = not dual channel
SLOT 1 + SLOT 3, SLOT 2 = not dual channel

thanks for the info. i've managed to get 2x1 gig sticks of corsair 1.9v which is great. 1.8v would have been an option but not what i was looking for. all 4 sicks in all 4 slots and seems to be ok. as i said earlier i've seen people populating all 4 slots with ram on the p5k and having trouble with bsod issues etc but have'nt had any so far. before i installed new ram i restored default in the bios in case the small overclock from 2.4 to 3ghz conflicted with installation, later i went back to 3ghz and it seems ok. will keep a check on things. just one more thing, i altered the ram timings slightly as well. ram info gives them as 5,5,5,18 so i changed to 5,5,5,12. should i expect a small performance boost or will it just be the same? also i'm interested in trying to overclock ram so can i leave my cpu overclocked at 3ghz but change my ram timings to 4,4,4,12? the corsair is 800mhz. the more i get into overclocking the more questions i seem to have, interesting stuff really.
cheers
cheers

just a quick update. mentioning the bsod issues using 4 sticks of ram, i did a memtest86 with 6 passes no errors. thought things were ok when pc gave a flash of bsod before randomly rebooting. changed the ram timings from 5,5,5,12 to 5,5,5,15, don't know if this will make a difference. should i raise the ram voltage by 0.5v and maybe raise the northbridge voltage a little? is 6 passes with memtest86 long enough or should i run it for longer?
cheers guys