I recommend that you don't borrow against your 401k, except as an absolute last resort and only in an emergency situation. 401k loans are convenient but they're not really such a good idea. Circumstances can change at any moment and, especially as a student, an unexpeted financial development may affect your ability to pay back the money on schedule.
Its one thing to think of it as paying yourself back with interest, but the other way of looking at it is that, depending on prevailing interest rates, over the long term you'll more than likely be losing out on higher market yields on the amount you took out of your account for the loan. This means the higher the loan amount the more money you'll likely lose over the loan period, and you'd end up with less money when you retire than you could have had. You also hurt yourself in an even bigger way when you take out a 401k loan because you must pay back the non-taxable principal with taxed income. Therefore, the IRS ends up getting more of your hard-earned income than they're really entitled to. And, there's more... If anything (except for a medical emergency) happens and you don't repay the loan amount within the stipulated time period you'll be facing a double-whammy financial penalty from Uncle Sam, which includes 10% of your retirement savings plus you must also pay income tax on the loan amount. Depending on your income tax bracket and the amount of money you borrowed this could make a huge dent in your retirement savings.
My situation will be very similar to yours when I start grad school next year. The advice I've gotten from people I know in the CRNA program is:
1. Go on the cheap. Cut back on EVERYTHING. (No cell phone, high-speed Internet, designer clothes, vacations, eating out, etc.). There's one guy I know in CRNA school who quit his full-time nursing job, gave up his apartment, and moved back into his parents' house with his wife and his son. His motto is "whatever it takes".
2. Look for scholarships
and grants. I've heard these are easier to get at the graduate level because there are fewer people applying for them. This may or may not be true but it doesn't hurt to start looking around from now to see what might be available.
3. Hit up your employer (or a potential employer) for a sponsorship. Many places have programs where they will pay some or all of the costs in exchange for an employment commitment after you're done with CRNA school. A two or three year employment commitment is a tad better than paying back a $150,000 (or more) loan.
4. Finally, look into getting the lowest possible interest rate loans from the federal and state financial aid programs first, and if that isn't enough then you can apply to a private lender. There's a lot of them out there competing for business so the money is there, its just up to you to look around for the best rates.
You should start getting used to the fact that you'll probably need to take out some major loans for CRNA school. From what friends in the program tell me, there's no way you'll be able to hold down a full-time job while you're in CRNA school, and you'll need a lot of money to cover your day to day living expenses for the entire time. These loan amounts sound kinda scary, but its an investment that pays off BIG TIME in the end.