Hi veeru 82 007
GMAT Prep is the new version of PowerPrep or vice versa. I am not sure which was the older, but you can chek in the official site mba.com by dowloading the last version. The diffrence is in the interface.
Regarding your scores, it depends which programs you are aiming to. If you would like to apply in top10, then you need result 700+. You can check the average GMAT scores for different universities on businessweek.com.
If I were you, I would try to improve significantly my score before I sit on the real exam.