Speaking as a practicing nurse who is now accepted into the 2015 class, I have a different perspective on this.
I initially did very poorly in college after high school, and when I decided to become a doctor I was advised by the premed advisor at my school to have a backup plan. I think she was politely being skeptical of my chances. She was also right, because based on my history I had no chance. I was also recently married and had a family to support financially, so I dropped out of the premed program I was in and enrolled in nursing school because I knew I could get in and I knew I could always have a job if need be. I did it fully intending it to be a stepping stone to becoming a physician in the end. In short, I did my backup plan first because I doubted my ability to do well enough in the basic sciences to compete in medical school.
Looking back, there was no way I was ready to be a doctor at the time. But working alongside doctors on a daily basis has given me a huge advantage (I believe) in how I am going to learn medicine. It is true that they are vastly different professions, but there is also plenty of overlap. I finished an associate's degree and then worked as a nurse for the next four years while getting a science bachelor's in order to apply to med school. In my case, it worked out. That being said, if you believe you can study well and earn the grades you need, don't sell yourself short -- you'll get in somewhere. Part of what hurt me at some of the schools I was really interested in was low grades because I worked full time while going to school.
In my case, I did it for financial reasons because of my family. You'll have to decide if your own reasons are sufficient. Just remember that you need to explain it to admissions, and if you sound flaky it will hurt you. If you are planning to apply to medical school directly on graduating, it may look flaky because you will never work as a nurse and therefore haven't benefitted from the clinical experience. You may as well double major in physiology and chemistry in order to wow the admissions -- they can train you in the medical stuff.
Also, doing a double major in chemistry may help an admissions as a CRNA, but not nearly as much as you might hope. CRNA admissions are very similar to medicine. They want good grades and a standardized test (GRE instead of MCAT, so much easier). But they also want clinical experience. Its nearly impossible to get into CRNA without intensive care experience.
Here's my recommendation: if you are planning your life in a way that will allow you to work as an RN for at least 2 years before applying to medical school, it may be worth it. Make sure you get a job in a surgical ICU and work your tail off at learning. Be wary of doing this while getting any other degree, because it will be hard to do well academically and that will automatically kill your chances at some schools. Then, if you don't get into med school when you apply, you've got the nursing degree and the ICU experience required to try for a CRNA position.
Don't try to rush things.
Do well at the things you commit to.
Do that, and you'll get exactly what you aspire to.
