I have this MB, Gigabyte GA-P35C-S3 (rev. 1.x) which it has 4 x 1.8V DDR2 DIMM sockets 1333/1066/800.

At the moment i have 2x 1Gb PC6400 DDR2 Kingston memories. Question: i want to put 2 more Gbs, with 2x 1Gb. Will i have a problem if they are from a different brand and/or with a different speed (1066/1333) and will there be a problem if the 2 new memories are at 2.2V while the slots are at 1.8V?

Thank you.

My understanding is that the RAM channels are paired. For the best performance, two RAM chips occupying the same color slot must be the same type for synchronous performance. I don't think it matters if the two pairs of RAM are different.

If one or more pairs of RAM chips is mismatched, I believe you can go into your BIOS and switch (or maybe the motherboard will do this automatically) to asyncronous mode. Asyncrony is slightly less efficient, but it works.

Voltage is tricky. In some instances, it will work ok. Other times your computer will reject it and not start at all. It depends on the specific products you're trying to use. I would say anything outside of your motherboard's recommendation is a gamble.