I'm wondering if the ACT has a curve. If it does, how significant is it?
Yes, the ACT is curved differently upon each administration of the test to account for slight variations in difficulty level and student performance. Most students shouldn't be concerned with this curve because the differences from year to year are relatively minor, and they can't be predicted ahead of time. The only way the curve might matter to you is if you're shooting for a perfect or close to perfect score. Sometimes, you can get two questions wrong on a section and still earn a 36, and other times you need to actually have a perfect raw score to end up with a perfect scaled score.
Based on the data in this chart, the biggest gap I found between two curved scores derived from the same raw score (not including curves for practice tests in the Red Book) was three points. If you got a raw score of 62 on the ACT English section in April of 2014, your curved score would have been a 26. If you got that same raw score on ACT English in December of 2015, your curved score would have been a 29.
It's nice to be aware that the curve exists, if only to set your mind at ease. A perfect score is not necessarily out of reach just because you answered one question incorrectly. My prep advice with regards to this is just to take a few real practice tests from different administrations of the ACT so you can get a better sense of the test's difficulty range. The curve shouldn't be a significant factor in your ultimate performance.