The hypertransport [inter cpu link] on the AM3 is wider than the AM2+ check it out at tiger or newegg under specifications or any good review site.
As for the video ,,some say that the AMD is optimized better for their cards rather than Nvidia,but that dosen't really make sense,that is from the pci-e view point as it is a standard to which the cards must be compliant,so maybe the diffirence might be in the chipset itself,I am currently running an 8800gt on an AMD 790 chipset and it seems to run just fine..
I got a 720BE for my 790GX. I don't feel at all compelled to replace it with an AM3 board. If I were buying new, I'd choose AM3 because it supports DDR3 RAM, but I wouldn't anticipate a big difference.