Well RAM really has a tiny consumption - regular modules should take like 2-3 watt each so nothing to worry about...
I've seen higher (depending on where you look). It depends on the sticks, the amount of memory per stick, etc. But, you are correct, it is not a great deal. However, the CPU needs power as well (duh!). The Q6600 can require 100 watts (or more) under a heavy load (not at idle). If you are running something heavy (like a decent 3D game, for example) and you have both the video card pumping and the CPU, then you begin to draw some serious watts. Add to this the RAM (as previously mentioned) and other little things that are drawing power and it all adds up.
power undersupply won't make your system slower but unstable.
From what I have read (and from talking to a few tech guys) it can. For example, using the Windows Vista Windows Experience Index we were able to bump the number upward from 4.6 to 5.0 on a particular PC by installing a larger power supply. The reason for he first number (4.6) was due to RAM speed (the Windows Experience Index gives the overall number based on the lowest number ... in this case the RAM was the lowest at 4.6).
because ram banks should be equipped symmetrically to take advantage of the dual channel architecture.
Yes, you are correct and two of them would be used in parallel. He could, if he wanted to, install two 1 GB sticks (in parallel) and two 512 MB sticks (in parallel). This would give him the three gigs and take advantage of the dual architecture as well.
