HI Marc,
I assume it was a 'typo', but in your prior post you wrote that you wouldn't apply for the GMAT until you had 4 years of work experience - I assume that you mean that you won't apply to Business School until you have that work experience.
When you say that you used the GMATPrep software, did you take the first practice Test/CAT? From what you've described, I think that you did. While those 2 CATs have limited 'pools' of questions, those Tests DO adapt to your performance. Thus, if you took the FULL CAT (including the Essay and IR sections), and did so under reasonably realistic conditions, then that 590 is accurate enough to assess your current skills.
1) What date did you take that CAT?
2) What date is your Official GMAT?
GMAT assassins aren't born, they're made,
Rich