Many--if not most--nurse anesthesia schools prefer surgical ICU experience, whether it's a general SICU or CVICU. Some schools prefer ICU experience in a Level I Trauma Center, regardless of ICU subspecialty. Certainly all anesthesia schools prefer ICU experience with high acuity patients and a good deal of nursing autonomy. Best to field this question to potential schools you're looking into for more specific advice.
This may not be exactly what you want to hear, but I would caution you against spending much time or focus on GRE stuff right now--your time and effort would be much better spent focusing on developing leadership qualities to put on your CV (committee involvement, volunteer work, etc.), earning professional certifications, and getting into an ICU. Then, when you're in the ICU, focusing your time and effort on becoming a strong ICU nurse (a pretty big learning curve in itself) and earning your CCRN credentials (plus other certs if possible). The GRE is a small piece of the nurse anesthesia school application, and while at least moderately strong scores are important, perfect scores won't offset weak professional experience, leadership skills, and clinical knowledge.
ETS, the company that writes the GRE test, has several books and study aids available that are reasonable priced. Of all the GRE study aids I had available, these were the most helpful. They also offer practice tests you can take to get a good sense of your baseline so that you can better focus you studying efforts. I supplemented with Magoosh and Khan Academy (free) to go over concepts I needed to better understand. Skip the review courses from Princeton, Kaplan, etc. unless you really need intense instruction on everything (and most people don't)--$1000+ is way too much money for a relatively minor part of the nurse anesthesia school application.