Stafford loans are currently fixed at 6.8%. I don't think you can find lower private loans. Federal Plus loans are fixed at 8.5%. Again, it will be hard to find loans for a better interest rate.
You get to pick your lender for Stafford loans and so in a way you get to pick your interest rate - different lenders have different perks associated with their offers.
You'll have to apply for your own Federal Plus loans to make up for what Stafford loans won't cover. Most likely the lenders you can pick for that will be the same as those you'll look at with your Stafford loans.
It depends if you have an established lender before you started college. Some lenders about 4-5 years ago gave out private loans with interest rates of around 5%. If you consolidated your loan, and let your lender know that you will use them for med school, some of them might even go as far as locking in your loans for med school at pre-inflation levels. Ask your current undergraduate lender and see if they can provide this for you as well.