Well I've been shopping for rims this entire month, so I think I've got the whole backspacing/offset thing hammered pretty well into my brain. 8 inch wide wheels with a 4 inch backspacing pretty much means that you have zero offset (the centerline of the wheel is exactly where the mounting flange is). You should have no problem with that. Zero or Negative backspacing is what you want for a Regal.
The wheels you have now, OEM 7" wide with 3.5" BS, basically the same thing. 3.5 is dead center of 7, zero offset.
Going up from 7" wide to 8" wide, with the mounting flange still centered in the rim, you're basically getting a half inch closer to the frame on the inside, and a half inch closer to the fender lip on the outside.
245/45/17 will keep your overall size very close to the OEM 215/65/15. The overall circumference will actually be LESS on the 245/45/17. You should have absolutely no problem fitting these.