|"the best option will be to get 2x2 gb 667mhz double sided, is that right ?"|
No! Your board does NOT support 2GB sticks. You MUST use 4 x 1GB single sided. Read what is in bold in response # 1.
Are you running Win7 32-bit or 64-bit? You should be running 32-bit on that system. Did you install a video card or are you still using the crappy onboard graphics? You should be running a card.