|Here's your board specs:|
Do you already own the board? If not, get something else. And I suggest that you stay away from Intel boards. If you want a board based on the Intel G41 chipset, look for G41 boards made by ASUS, ASRock, Gigabyte, MSI, etc.
To answer your question about the RAM, there are several Core 2 Quads that run at 2.66GHz. Some run at 1066MHz FSB, others run at 1333MHz FSB. And according to the above link, the board supports "DDR2 800 or DDR2 667 MHz DIMMs". If you have the 1066MHz version of the CPU, The "optimal" RAM would be DDR2-533 or DDR2-1066. If you have the 1333MHz version, the "optimal" RAM is DDR2-667.
However, if you don't care about the optimal configuration, you can run either DDR2-667 or DDR2-800, whichever is cheaper.