Well, the E4400 isn't going to be that far from the E4300, so the ratings for the the E4300 is close enough to be used, imo.
Here's a link you might look at (doesn't have X2 5600/6000+, but you can use the E6420 as a close example for them).
X-bit Labs review
If you don't overclock, you can always do a little research on how to do it. Most of it is done via BIOS, but some motherboards have programs that allow you to overclock through the OS, as well. Just do it slowly, and a little at a time, for the best stable system you can achieve.
Your items are good for a system. The Gigabyte motherboard does have a program called eTune that will allow you to overclock, although your memory might restrict the amount of overclock you can achieve. Otherwise, it looks like a solid build.