I am a big believer in getting it right in the camera, and being light in post. That said, sometimes you have to do post processing.
I used Gimp and Darktable when my main machine was Ubuntu. I had to get a MacBook Pro for my day job (I write software) so I picked up copies of both Aperture and Lightroom 3.
Since then I use Aperture. I'm tempted by some things, like lens correction that LR3 has and Aperture does not, but the basic workflow of Aperture appeals to me.
I found that Darktable was a cool idea, but too immature. It was so slow that it constantly got in the way. And it would periodically just crash. It may be better, as I haven't used it in 10 months or so. But for me there was no going back.
I don't do much that requires the kinds of pixel tweaking that you can do in Gimp and Photoshop. I consider them to be graphics editing tools, and I look for photography editors.
I do the usual adjustments to exposure, details, saturation, etc. Any tool will do that, and most do it with minimal fuss. Crop and straighten, and minor retouching.
I fully believe that using either Aperture or Lightroom will let you be happier with the results. Of course, Aperture is Mac OS-X only, and Lightroom is Mac or Windows only.
Aperture is $80 from the Apple store, and LR3 is on constant specials for about $100 as LR4 is to be announced in a week or so. If you run Windows, get LR3 and take advantage of the upgrade special price that is sure to happen once LR4 is out.
There is really very few other places you can spend $100 and make as much of an improvement.
I recommend you read the Strobist blog, he has lots of specifics on how he takes shots, including a large number of shots taken in the golden hour and augmented with flash. You can get a nice Strobist starter setup for $300 from MPEX