> Once thing I alread notice with the d5000 is that auto makes the pictures too soft
Cameras don't do that, per se, and neither does an auto mode. What it may do is choose a shutter speed not entirely appropriate for the subject, or you might not have the AF quite nailed.
> I noticed a few shots I took at dusk fo the kids playing turned out ok, but there was a fair amount of them that were blury.
That's almost certainly because the shutter speed was too slow for the subject. In general the camera doesn't know the characteristics of the subject, so it can't adjust accordingly. You still have to do that.
> I prefer programmed mode with the setting on vivid
Unlike the various types of film, digital sensors tend to be very, very neutral. The vivid setting corresponds to something like Fuji Velvia, which is wonderful for nature shots, especially flowers, sunsets, etc. But you may find that it's not so ideal for, say, portraits or weddings.
> Another addon I was looking at getting is a SB-600 flash, I'm guessing it's a huge improvement over the built-in flash in low light scenarios and indoors.
The SB-600 is quite a bit more powerful than the built-in flash.
> Just the ability to bounce the light off of a ceiling must help a tonne.
Indeed, bouncing off a ceiling (or a wall) is vastly preferable in most cases. Not only do you get much softer lighting and no direct flash shadow, it's also much easier on the subjects since the light doesn't blast them straight away. This is especially beneficial with infants and toddlers, who don't really have an understanding of what flash is any why it isn't a threat.
_____ Brian... a bicoastal Nikonian and Team Member
My gallery is online. Comments and critique welcomed any time!