Ram capacity of my computer

Have you tried Crucial's scanner? I'm sure it will give you a more definitive answer. Alternatively, try booting with only the 2GB stick, and run memtest to check stability. Most linux distributions with a LiveCD come with memtest on the disc as well.

"Dell says that the system can only accept 1GB RAM sticks

This means that Dell has tested the system with up to 1GB RAM sticks and that they know it works. It does not always mean that later introduced (larger) memeory sticks will not work. It just means that:

It was not tested at the time the manual was written.
It may be a hard limit from the memory controller, lanes to the DIMM sockets, etc etc.

A side question, I hear about RAM requiring about 15W of power per 1GB, should i be worried about my system not having enough power if i upgrade?

That sounds very high. I see about 2W per 4GB added. GDDR for graphics cards uses a lot mor epowr though. I wonder which figures you checked."