Re: GMAT !
like X2 said... there's a whole bunch of factors that determine whehter a score is "good enough" to get into the program of your choice. Generally speaking, 650 is considered safe... and 680+ is considered pretty good.
To answer your second question, yes... I did go for the GMAT without any prior prep just with the intention to see how difficult the exam is... and I scored 660 at the time. What I figured is that if I had taken the time to go over some tips and tricks for answering the questions more efficiently, I'd have scored much better. The only difficulty I faced was time management... the questions themselves were not difficult. For anyone who has gone through the University system in the West, the exam isn't hard... you just need to learn the right approach to answering questions.
What many schools do however is pay particular attention to your scores on the Analytical Writing Assessment (AWA) component which is scored separately and this component may require additional training especially if you haven't written essays etc. in a while.