|Cpu xternal speed (fsb) and memory speed should be equal (ratio 1:1).|
U have q6600 which is 1066fsb.
underclock ram to 533ddr.
Some motherboard can run dual channel with 3 sticks, some can't, some can run with different sticks, some can't.
U should be able to run dual channel with your current memory (2x1gb if they are identical)
i can't tell u if u can run dual channel (after mem upgrade).
If u mix mem (1gb ddr2 800 and 2gb ddr2 667), 1gb ddr2 800 will run at 667ddr.
(4x1gb or 2x1gb and 1x2gb=4gb 64bit os required)
edit: try to compare timings and mem voltage (may be u can run dual channel)