GT300 core is a ways out. DX11 is featured in Se7en and I see it taking off in much the same way DX10 did. It's nice to have and it offers a bonus in performance but will not be a necessity for a few years to come. The GTX260 Core216 is a good bang for the buck card at 200 dollars. I haven't priced or spec'ed out the 58xx cards lately, so I'm not quite sure.
@ ShozzKing, the RAM you suggested is pretty well perfect for that system. 1.65v, and good timings.
But the case is ridiculous. And I would never trust a PSU that comes with a case unless it's a NewEgg combo. But coming together in the same package? No way.
(There are a few others, but I personally love this case)
You see, if you pick up these two items from Tiger, you get free shipping (assuming you are within the 48 mainland states).
EDIT: I kind of posted this PSU to introduce you to the available items that Ultra makes in the way of PSU's. 38 amps is fairly good, but also kind of low for a 650w. Once you have a final decision on the GPU's you will be sporting, then you decide on how much power/amperage you need on your 12v rails.