DVD and speaker options
For DVD players, check out Oppo Digital. I am impressed with the PQ of their budget DV-970HD player and it will also play DVD-Audio, SACD, and photos from my SD memory card.
Don't buy Sony or Onkyo speakers. There are plenty of better options depending on your budget. It is a good idea to buy all of the speakers from the same product line for timber matching, but the sub can be from another manufacturer.
The only advantage I can think of for buying multiple components from the same manufacturer is that the front pannels will match. If you insist on buying a receiver and DVD from the same manufacturer, then go with Denon.