For your motherboard, you either put them in the slots labeled DIMM1 & DIMM2, or in DIMM3 & DIMM4. The RAM sticks have to be in slots directly next to each other.
I put them in DIMM 1 & DIMM 2 as suggested, turn on computer, run CPU-Z application and in Memory tab I see that there is 2048MB of DDR2 RAM memory but it shows single channel. Is there any crucial setting to be changed in BIOS?
Tnx
That's odd, the manual shows that should have worked. If you haven't done so already, try putting them in alternating slots (1/3 or 2/4) and check what CPU-Z reads.