Memory controller on AMD Phenom is different from Intel's.
How scales different speed/latencies for ddr3/ddr3 on Phenom's ?
How to find wich is better, more cheap ?
Better 2*1024Mb dual channel OR 3*1024Mb SINGLE ?
Um, I didn't understand much of what you said sorry, but, there is not much of a performance difference between DDR2 and DDR3 using Phenom II processors. I don't know how much of a difference there is between AMD and Intel processors, in regards to DDR2 and DDR3 scaling.
To find cheap and good RAM, just look at a few reviews, and higher speed is well better, and lower latencies like 7-7-7-20 is better than 8-8-8-24, for example.
As for the last question, 2GB in dual channel will be slightly better than the 3GB in single channel where you do not need the extra RAM, but need the extra bandwidth. However, 3GB of RAM would be better overall, as it would provide more RAM for general use.