One thing you will soon realize when it comes to the best price/performance is that there are different sweet spots as you go up the budget scale.
However in the 400-600 dollar range, things such as Black Edition CPUs, aftermarket coolers, Crossfire motherboards and super-beefy PSUs (to support all these extras) tend to eat your budget up very fast. Leaving a lot less for everything else, and ultimately coming out worse.
For example here is a $600 build with a Crossfire Mobo, Black Edition CPU and aftermarket heatsink.
This following build is a good compromise since it still has a crossfire mobo which will let you build off when you choose to upgrade later, but also a way better video-card.
And ultimately if you want to stay within your budget instead of on the edge, I suggest the following.
(Note: While the motherboard may have a second PCI-E slot it is only x4 not x8 so it isn't useful for crossfire.)
The Seasonic PSU is definitely one of the best in its price bracket, same with the Samsung Spinpoint.
The Cooler Master is a great case as well, the only other ones I'd recommend would be the NZXT & Antec cases for the $40-60 range.