Mismatched RAM should not be a problem with a G41 board. The G41 has a limited FSB range (compared to the P45's at least). Upper limit, depending on the individual board, is around 360 MHz.
This board, because of the limited BIOS settings is pretty easy to overclock. (I know; I have 3 of them.
Shadow's Gigabyte motherboard OC guide:http://www.tomshardware.co.uk/forum/page-245679_11_0.html
It's for an EP35-DS3L but all the Gigabyte Core2 BIOS's are similar.
The following instructions are for a Gigabyte board, but the Asus board BIOS shouldn't be that much different.
First, take the memory settings off Auto and for a DDR3 board, set the System Mem Mult to 4.0. At stock CPU speed, your mem clock should be at 1066 MHz. This is important. Once you have this, the rest is simple.
Then take the CPU voltage off Auto. Most Q6600's will run at 3.0 GHz (333 MHz X 9) with little or no core voltage increase. So boost the FSB freq to 330 MHz and increase the CPU voltage for stability if necessary.
I recommend increasing the FSB in 5 MHz steps until the system does not run. Depending how lucky you are - well, how good the board is, you will reach a stopping point at around 340 - 360 MHz.
A DDR3 memory controller generates 4 clock cycles for each bus cycle, so the frequencies shown in the CPU-Z SPD tables are actually doubled. FSB is halved and RAM clock is doubled.
You do not need to worry about the memory voltage. The original Core2 CPU's use an external (to the CPU) memory controller. You can go up to 1.65 volts with no problems.
The limiting factor of your overclock will be the motherboard.