Bunch of questions pertaining to an "overhauling" of my system

Hello guys. I am preparing to do a major upgrade to my system by fall 2011 (as in new motherboard, memory, graphics card, and processor).

So, basically, what I want for my system is:
a quad core processor,

at least 6 GB, ideally 8 GB of DDR3 memory

and a graphics card that will give me very good performance on 1680x1050, maybe 1920x1080.

So I've done some calculations and new parts and the new monitor, if bought today, would cost me about 650$. So a few questions
1: Can I expect DX11 cards to fall much between now and Fall?

2: What is the advantage of DDR3 over DDR2? I currently run 4GB of DDR2-800, and the only advantage I see is that there seems to be a lot more memory per "stick" on DDR3 memory. What should I know about DDR3?

sorry if my post was a little disorganized.