I have the D600, and itís a great choice when I need to fly out a heavy and bulky light/modifier several feet from the stand.
The Impact stand you presently have or a C-stand will handle the D600 boom, but I feel they would be a little light if you were to load the D600 with the weight and counterbalance itís capable of carrying.
When I have a situation that requires the D600, I prefer a wheeled stand with a larger footprint so I can move it about easier. The Avenger A5036CS works well, and is easy to sandbag.
For less strenuous setups, you might want to consider a baby combo-boom stand like the Avenger A4041B. Iíve never used this stand, but it looks interesting.
hmm how do you think the Impact stand would go if loaded down with lots of sandbags?