Nursing assistants will not replace Physician's assistants, at least not here in the US. A phys. asst. (PA) functions like a Nurse practitioner here. Nurse assistants are usually nurses aide's (CNA's) etc. The main difference between a PA and an NP is that in many states an NP can have their own practice, whereas, a PA always has to practice under a licensed MD/DO.
Many NP programs require you to have at least 1 year of experience as an RN, some even require more, like 2 years, or require acute care experience. It all depends on the program you are looking at. One note to consider is that by 2015 the ANA theoretically wants NPs to become DNPs, doctor of nursing practice, in order to work independently. According to my school this change will happen in or around 2015, so that might influence your decision as well. Most PA schools are 24-27 months but are highly competitive, like CRNA schools, NP schools etc etc. Hope this helps.