How many Pokemon Dream World Points do I need to see a legendary?

Unfornately that is not true. The Only Legendary (so far though) you can get through Dream World is from the Global Link Poll that happened last month. That Event is to happen in February 2012. However you can get rare pokemon such as Bagon and Dratini after 10,000 DWPs with 8 gym badges in place. The best pokemon you can get from there is Lapras, Dratini, Bagon and a few others.

The Only way is by events.

They might add an area where you can find pokemon starters and other really rare pokemon but not legendaries though.

First off, only a select few Pokemon are available in the Isle of Dreams in certain areas. You need to get a certain amount of points to see the hidden Pokemon in the respective areas. No legends are being distributed at the moment. Legends will only be distributed when Nintendo/Gamefreak feels it should be released, like with Arceus, how sadly many people voted for use to get it using the Global Link so it'll be distributed in February, but Dream Points aren't required. Raikou doesn't have its own area to be found in so you can't find it. Check www.serebii.net to see who the Pokemon available through the Dream World are.