Hi!
The EFS 17-85mm is useless handheld in lowlight IMO, even with IS (which isn't the best example of IS anyway), it's still a slow lens.
I think for the best chances of taking handheld shots in low light you do indeed need faster glass, whether it's f\1.4, 1.8 or 2.8.
Faster prime lenses like a nifty fifty or 85mm can help you if the focal range is suitable for your needs.
IS may help but also and quite importantly, practise how you hold the camera for the best results.
Check out Joe's tip:
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Using this tip combined with alot of practise, I managed to shoot at shutter speeds I never thought possible. After alot of practise I got a crisp shot at 1\8th! I kid you not!
It's not easy, especially if you can't get used to using your left eye and it also works better for me with shorter lenses. Never the less it really works!