We use Watermarkmaster from Videocharge.com. I use it on my pc and also on my server. It will suck the tar out of your cpu no matter what size. The more cpus the faster it runs but will still hit 80-90%, which is fine as long as you have a decent cooler. While its running you pretty much cant do anything else.....even on a server.
We bulk watermark on our server one day then submit the other 6.
Hope that helps.
