Your motherboard specs are here:
http://usa.asus.com/product.aspx?P_ID=yQYfAtkxxZMjR79G It supports 8GB. This would be 4 - 2GB modules at 800MHz max.
I couldn't find your Corsair model with a "PRO" on the end. But it's likely this:
http://www.corsair.com/_datasheets/CM2X1024-6400.pdf
If so, note the important spec of 1.9V. If you can match this and the other specs, no reason you can't try system at 6GB.
This is the 2048 version of your module:
http://www.corsair.com/_datasheets/CM2X2048-6400C5.pdf
Or in the 2 module kit
http://www.corsair.com/_datasheets [...] 6400C5.pdf
The specs on the RAM you have are good enough to not toss them out. You might be able to OC them now to a CL=4 anyway.
I'd try adding the 2 new 2GB modules and see how you like the system. There's always a chance the motherboard won't like having 2GB and 1GB RAM in at the same time, but not likely as long as voltage is the same or close. If you're running all those functions on Auto now, reading the SPD, I'd say it'll all work fine
But but but... you're running Vista Home? Isn't this a 32.bit version of Vista? If so, it's not going to address more than just over 3GBs anyway - with 4 or 6 or 8GBs installed. Need a 64-bit version of Vista, or Win7, to take advantage of more than 4GBs of RAM.
From what I read, video editing would enjoy having LOTS of RAM.
Sounds like it's time to investigate upgrading to Win 7.