Which is higher magna or suma cum laude?

Depends on schools. Ussually it is...

Summa Cum Laude is 3.9 and better.

Magna Cum Laude is 3.7 - 3.9

Cum laude is 3.5 - 3.7

There is also Egregia Cum Laude which holds the same GPA requirement as summa but is only given to the selected elite.

http://en.wikipedia.org/wiki/Cum_laude