GMAT Strategy

Hi,

I am taking MGMAT and GMAT prep practice test.
I am scoring in high 500's to low 6OO's.

On MGMAT tests I feel quant section was very tough.
In fact I could solve tough quant problem if I had some more time.
On one of the MGMAT test 120 line RC threw my timing off.
On MGMAT I am receiving most of the questions in 600-700 range
I am planning to work on questions on 600-700 range
to improve my score.

So far I am practicing with MGMAT and GMAT prep test.

Does any one recommend any other practice tests?
Are MGMAT tests a good measure of GMAT preparation?

Do I take any other tests like KAPLAN or ??/

Any suggestion on attacking problems in 600-700 range?

Any advice and comments will be helpful.

Vijay