In the video area of the PSN Store on PS3 there will be 800 films at launch. Some will be just to buy, some just HD and some just SD. Release dates will be the same as in the shops.
Of those 800 films, 125 will be available in HD. The plan is to release 50 film movies onto the store each week. As time goes on those new titles will replace the less popular ones. You will be able to search the store by studio as well as finding the films grouped in collections, such as finding all the X-Men films together.
For those of you interested in knowing the size of the downloads one example given is the HD version of the film 300 which will be 6.6GB in size.