I would invest in a good NCLEX review book just to have and then use it to practice for the HESI. I have Saunders (blue cover) w/ the CD. It is a great book. I use it to study for my tests as well.
The HESI is "supposed" to be a good indicator if you have gained enough knowledge to pass the NCLEX, however, most people will tell you that they've failed the HESI and passed their NCLEX w/o issue.
We take the HESI 4 times during school and why we take it is to measure the instructors to ensure they are teaching us things that are going to be on the NCLEX. We are expected to fail it the first 3 times we take it, because obviously we haven't gained enough knowledge our 1st 3 semester to pass the NCLEX. They get a little upset though if you fail it your final semester because by then you should have the knowledge. However, it's still a tool for the instructors to figure out what is left to cram into our already overfilled brains our final semester.