Looking for recommendations

I am building a new PC for the first time in 4 years. The machine will be used for streaming video content (DVD movies, recorded TV shows, Hulu, Netflix, etc) as well as editing AVCHD video. My budget is $900 but I would like to keep the cost as close to $700 as possible.

I have selected the AMD Phenom II X4 940 as my CPU and I was looking for a good motherboard recommendation. I need to support a dual monitor setup (preferably both DVI) if the mobo has that, great, if not I will just add in a graphics card. Recommendations?

I am also kicking around the idea of 8 GB of RAM for this machine instead of 4. However, that forces me into a 64 bit OS. Is the extra 4 GB of RAM actually going to be worth it?

From what I have read and others have posted it seems I really need to go with a 780G or 790GX chipset for the HD decoding. That being the case I am looking at all of these:
ASUS M4A78 PRO AM2+/AM2 AMD 780G
ASUS M4A78T-E AM3 DDR3 AMD 790GX
GIGABYTE GA-MA78GPM-UD2H
GIGABYTE GA-MA780G-UD3H
GIGABYTE GA-MA790GP-UD4H

Is there any real difference between 780G and 790GX in terms of HD decoding? Those boards are about $30 more than 780G boards from what I can tell.

From what I can tell I can do dual monitor output on all of them.