First of all, there is no minimum score in GMAT if you do not know the type or types of business schools you want to attend. Some business schools will tell you their minimum GMAT while others do not have a minimum GMAT score but will tell you the higher your GMAT score is the better.
Second, you mentioned that you got a 451 GMAT score on the first CAT exam and you are asking for tips on improvement. Before I answer this question I want to know what is your weak area? If it is math then I suggest you to purchase the following math books:
1. Math the easy way from Barron's. You can order it via amazon.com.
2. How to Solve Word Problems in Algebra, (Proven Techniques from an Expert). You can also order it from amazon.com
If your weak area is verbal then I suggest you visit the powerscore.com since this company has very good books on critical reasoning as well as sentence corrections.
Third, go to
www.mba.com and download the CAT software program because I believe that the program will be able to evaluate the AWA after you finish it.
To improve AWA score I recommend you write multiple essays no more than 3 each per topic.
Best of luck to you,
GMATFREAK