You should experiment with using the ISO on your camera to deal with low light situations first. It won't take long and it's like getting a free upgrade, and most important, you really need to understand your camera's
REAL limitations so that you can be more effective with it. In a typically "dark" situation indoors that you now want to solve with a faster lens, crank up the ISO incrementally with your current lenses to get faster shutter speeds, and then review the photos to see where your acceptable threshold is - not at 100% pixel-peeping level either - at regular viewing size levels. You might be surprised how well they'll turn out.
When you've established where exactly your noise gets to the point where it's unacceptable you're armed with the actual info, and can then work with it. You can work up to that threshold without worry, for one. But it's also important to understand that the minimum amounts of noise generated by cranking the ISO that just cross over that threshold for you can be dealt with very effectively in post processing with software. Lightroom has a good noise cancelling module and 3rd party software like Noise Ninja works great.
You've already got a pretty good range, from 18mm to 250mm. If you want more, especially for wider shots indoors, you're looking at one of the ultra-wides in the 10-22mm range. Canon's 10-22mm is a terrific lens and fits into your budget last time I checked. Third party variations on that from Sigma and Tamron are said to be quite good as well from their users, and will save you some money.
I recommend to new shooters that they work on their shooting technique and post processing abilities before buying more lenses, unless they need focal lengths they don't have covered yet by their current lenses.
See this thread as a reference:
http://www.thephotoforum.com/forum/canon-lenses/324746-prime-lens-dilema.html#post2931820 I think it might help you understand where I'm coming from.