The way ATI's branding works, Radeon 4x70 will always be the highest card in the generation and  the bigger the X is, the faster the card is.

There were some rumors about Radeon HD 4x90 cards, but we can tell you that there won't be any announced, at least not for the American or European markets, while Chinese market tends to have a mind of its own.

So, the highest performing card in that generation has 70 at the end of the name, the mid range uses 50, while the entry-level part ends up with 30. It's quite logical and the bigger number is always better where Radeon HD 4870 will end up faster than current HD 3870.