Hi Ignatius,
First off, a 620 is a strong initial CAT performance (the average GMAT score hovers around 540-550 most years).
From what you've described, you're clearly "rusty" when it comes to math, but many Test Takers are too when they start their studies, so you're not alone. Since you claim to have performed well on the Quant sections of the SAT and ACT, it stands to reason that you could probably train to score well on the Quant section of the GMAT.
Before I offer any specific suggestions about how to maximize your studies, I'd like to know more about your overall timeline/goals:
1) What is your goal score?
2) When are you planning to take the GMAT?
3) When are you planning to apply to Business School?
4) Have you purchased any GMAT resources yet?
GMAT assassins aren't born, they're made,
Rich