Adding memory and bios upgrading

Question

I have 512g ddr2 533 mhz of an unknown brand and i want to buy one giga. I don´t know if it is preferable to use the dual channel possibility and remain on the same speed with 1.5 Ram or if i should instead go for 1 g of 667. I don´t really know the differences and potentialities of the dual channel but i read in the manual it was obligatory to buy from the same brand and i can´t even tell which brand it is. On the other hand i have this Pc since 2006 and i guess i should make a Bios Upgrade which i also distrust a little bit to do it on my own because someone told me it could be problematic. How hard is it for this motherboard?