Yes, Busch Gardens is roughly 30-40 miles from Va Beach, but add time for traffic. Kings Dominion is 20 minutes north of Richmond, so much further away. If you want to rent a beach house, then a week would be nice.
However, if you just want to hit the aquarium, have 1 beach day, 1 day at Busch Gardens, I'd rent a hotel for 3 nights.