I was an RN in L&D/ Women's surgery for 4 years before entering medical school... now in 3rd year. For BSN, at least in Texas, you have to take gen chem 1/2, plus several biology classes (a/p, micro, pathophys, etc). When I decided to go back to school, all I needed to take was OrgChem 1/2, Physics 1/2, and Calculus (which I didn't really need because my statistics class counted everywhere I applied but UTHouston). I've known too many NP's having to work regular nursing shifts on the weekends to make enough money to pay their bills. The experience nursing brought to the table really helps, but if you know for sure you want to go to medical school, go straight through. You can get a BSN, and make decent money picking up shifts on the weekends through the first two years (if your study habits make that possible).
Good luck!