Moving into the top 5 is basically impossible and even getting into the top 10 is tough. You can look through past drafts to see that:
http://prosportstransactions.com/hoc...tTrades/Years/
Here's a list of trades that involved a team moving up in a draft that involved a top 10 pick since 2000:
2008:
-2nd and a 3rd to move from 7th to 5th
-2nd to move from 9th to 7th
2007:
-2nd and a 3rd to move from 13th to 9th
2005:
-2nd and a 7th to move from 12th to 8th
2004:
-2nd to move from 8th to 4th
2003:
-3rd to move from 3rd to 1st
2002:
-4th to move from 10th to 9th
So it hasn't happened in almost 5 years and even when it has happened the movement has been very small.