If you are building this machine specifically for editing using Vegas, then a faster CPU would make the most difference. 6GB of RAM is enough and Vegas currently doesn't use the GPU of the video card. This could change soon though.
Personally, i'd go with the absolute cheapest video card you can lay your hands on (under $10, or use your old one for free) until we know for sure which video cards work best. That way you've wasted almost nothing.
"Personally, i'd go with the absolute cheapest video card you can lay your hands on (under $10, or use your old one for free) until we know for sure which video cards work best. That way you've wasted almost nothing."
If Vegas is the only video application you will be running, then yes, just go with a good basic video card. But if you are using other apps, then verify if they can utilize any of the features of a video card and build around that.