It all depends on how much you're wanting to spend.
You could stay on the A-Mount system and get an A35/A55/A57 or A77.
The A55 has the best ISO performance out of them but the worst battery life.
The A35 is an A55 without GPS
, a flippy screen, and slightly worse ISO performance but comes with much improved battery life.
The A57 is pretty much an A35 with flippy screen and new body. It's the successor to the A55 but seems to have more in common with the A35.
The A77 is a much more professional body than the other ones. It's weather sealed and I believe has the best battery life. It does have the worst ISO performance due to the first generation 24mp APS-C sensor. It also has the best Autofocus (below the A99).
If you want to switch systems (I wouldn't advice it due to the cheap but great Minolta glass), I'd go for a Pentax K-5 or Nikon D7000. Both have the same sensors but the K-5 has better dynamic range and build quality.
The Nikon will have better video and autofocus. In the end though, it depends on which one you like more. Pentax is great at prime lenses but not so great at zooms. No Full-Frame upgrade path for Pentax either.
Since you're wanting to do lower light shooting, I can't recommend a mirrorless camera like I normally would.
They haven't quite nailed the low-light focusing with them.
Out of the cameras that I listed, I've only used the A55 and an A77 at a Sony store.
I like my A55 but I'm looking to move back to the NEX system (mirrorless) due to the size and becaus
Another suggestion of mine would be to head into a store and get a feel for what you like.