Wei,
Not many people are going to be able to respond with details regarding Forest Hills, Flushing, and Long Island City unless they live in Queens.
I live in Long Island City. Even within Long Island City, prices really can vary. If you are near the waterfront on Center Blvd, there really aren't single family properties for sale anymore - everything is sold as a potential development site. Every investor wants to buy there, especially since a modern 1 bedroom apartment rents for close to $4,000 per month today. However, there are multi-family properties for sale near the 21st Street Queensbridge subway station that sell for around only $1 million, because it's located near the government housing projects.
In Flushing, there's been a push into buying condos rather than individual houses in the past 10 or 15 years. What's driving up the prices in Flushing is as much cultural as it is the dense population. Since the Chinese population there have to purchase a home for their families, and borrow money up to the brim, they competitively bid up the prices of homes in the area. Fortunately the market there is strong and has been rising every year.
Both Long Island City and Flushing are appreciating areas, but it is very, very expensive to buy if you just want to occupy it yourself. As an investment, you're looking at 3-4% cap rates. I do not know much about Forest Hills. Good luck on your search.