Ok, you have alot of questions, so i will try my best. First your memory usage problem..most people seem to forget that your running your operating system (Windows Xp or w/e). Plus you have small things in the back ground. Also i dont know what type of processor you have, but im guessing its slow.
Second question, i cant tell you if you can transport your RAM if i dont know what mobo. So i cannot predict that the mobo you will get will accept your RAM. But for your present computer (notebook) im pretty positive you dont have dual channel RAM, so you can most likly put a 512mb with a 256. I would call your company and ask if your mobo supports that much RAM first.
Final, for your computer spec's, go to google or yahoo type CPU-Z and DL it. This will tell you all your computer specs.
Finished With TF.