Techpowerup talked to AMD about the problems they encountered with some Radeon HD 4830 cards that only have 560 out of the 640 shader processors activated and received a fix:
AMD on its part quickly followed up the issue with its engineering department, and released a video BIOS update that fixes the issue and makes available all the 640 SPs. AMD will be circulating this BIOS to all its press contacts and add-in-board (AIB) partners, to make sure the issue is isolated and fixed. The updated BIOS file can be downloaded from here. AMD also asserts that the issue isn't hardware related, and that updating the BIOS resolves the issue completely.