You need a lawyer immediately.
Amarillo is split over two counties, Randall and Potter. As a former Assistant District Attorney in Potter County, I can tell you with some authority that your case will be taken very seriously. With a first offense you have an opportunity to ask for probation, but recognize that without a skilled attorney negotiating your case, you are facing a very steep uphill battle when considering your quantity of more than 400g.
Both counties have given probation in circumstances such as you have described, but not often.
The rules prohibit us from making recommendations for specific attorneys in this questions section, so I will simply say this:
Hire an attorney there with experience defending cases in both counties. Hire someone with a track record of going to trial when necessary. You want someone who knows the court rooms well and knows the prosecutors by name.
You need to hire a lawyer immediately. Many times with drug cases there are search and seizure issues. Your case needs to be reviewed to see if there is a possibility of getting the evidence suppressed. If not, then you will need help in getting a deferred adjudication probation on the case. While you qualify for probation, this is a very serious charge and not one that they will simply hand out a probation offer on.
Be sure to hire someone with experience with drugs cases and suppression issues.