I do not really think that the video game series is vital to the understanding of the movie Silent Hill.
In fact, concerning the Silent Hill games, there are several of them, but they all deal with different stories.
For instance, the 1st game Silent Hill deals with a guy named Harry Mason who searches his adoptive daughter Cheryl after a car accident, while the game Silent Hill 2 talks about James Sunderland who decides to go to Silent Hill town after receiving a letter from his deceased wife Mary.
However, even though all the games have different stories, they still have common things like:
the main characters need to survive against a lot of creepy monsters, in a scary alternate dimension (for information, only some game scenarios occur in the town Silent Hill, but not all of them)
the player need to solve crazy puzzles
short vision range: sometimes when exploring the games, you have thick snow and / or no light (so, the player need to use a flashlight and BOOH! Here comes a creepy monster ;P )
ambiguous endings: according to the paths you followed during the games, you can have a "good", "bad", or "joke" (= crazy funny ending which can include UFOs for example) ending.
When playing a given game, you may find some references to another game, but these do not consist in a essential link to continue your quests and to succeed in the game.
When watching the movie, you notice that the adaptation shares similar stuffs coming from the video game series, but the story remains different.
Considering the end of the movie, I think that they wanted to keep this ambiguous feeling and let the spectators decide what would be the ending. So, it would be up to us to decide what happens: they may be still trapped in this hell dimension, or they might be possessed, or they might be trapped in their mind or some kind of nightmare (I think that in one of the video game, there's an ending where you finally understand that the main character was actually insane or in a coma... I will have to check that.