Not to start a war here or anyhting. but after a couple of years compositing on various projects I have to say that combustions main failing is its speed and stability. I have absolutely no loyalty to any package. I will happily use whatever works. I really want to use combustion as it has great features, awesome tracker and paint, 3d cameras and a huge promise, discreet master keyer and colour warper, it just has bit me in the ass so many times.
I have used it on a number of projects and on the mac platform its performance is dire. PC is much better and with a spec’ed out machine like you are talking about you should be fine for SD work.
I have found shake to be must faster and more stable on every platform i have used it on. I also have found after effects to be superior in its use of the same plugins. I would also urge you to have a look at digital fusion, I have only had some small experience with it but since you are basing yourself on PC and having seen version 5 (due out soon) I am very impressed with it !