I have a Canon EOS 40D, which is a bit outdated but it still serves me well. I'm trying to save up for the 7D, which I believe is what BJ Nemeth now uses. If I ever get my Full Tilt money back -- insert laughter here -- that's where it will go.
Given your interest in low-light photography and your price range, I recommend a Nikon. From what I've seen, the lower-end Canons (the Rebel series and 60D) don't handle noise very well, which is an issue if you're doing night shots. I'm a Canon guy but I do tip my hat to Nikons in that regard.
At your price point among the Nikon family, you're looking at either the D3100 or D5100. The D3100, which came out last year, is Nikon's lowest-end DSLR. The D5100, which streeted this year, is a step up before you get into the "prosumer" class.
I haven't followed the Nikon line as closely as I have the Canons, but here is DPReview's take on the D3100. I skipped to the "Conclusion" page as it provides a solid overview of their otherwise exhaustive tests:
Nikon D3100 Review: 19. Conclusion: Digital Photography Review
And here is that site's review on the D5100:
Nikon D5100 Review: 22. Conclusion: Digital Photography Review
Note: the D5100 is essentially the same camera JonV uses.
Just avoid those package deals that come with a lot of accessories. They tend to feature a lot of cheap junk. For example, when I bought mine, it came with a couple of really crappy lenses, a cheap case, some filters, a flimsy tripod and cleaning supplies. I've used almost none of those extras. I went into detail about just such a package in this
post from an old thread from when scooter sought advice on a camera. It gives you an idea of how much crap they give you in those packages.
If you go with Nikon, this looks like a very solid buy in your price range:
Amazon.com: Nikon D5100 16.2MP CMOS Digital SLR Camera with 18-55mm f/3.5-5.6 AF-S DX VR Nikkor Zoom Lens: Camera & Photo
If you opt for Canon, the equivalent of the D5100 is probably the Rebel T3i:
Amazon.com: Canon EOS Rebel T3i 18 MP CMOS Digital SLR Camera and DIGIC 4 Imaging with EF-S 18-55mm f/3.5-5.6 IS Lens: Electronics
As for other brands, I unfortunately don't know much about Olympus, Pentax or Sony. Olympus has its Evolt line of DSLRs, and Sony has the Alpha A33 and A55 models, but I've never seen them in use nor have I met anyone who shoots with them.