You can't just put any bios from one card to another one of a different kind and expect the card to continue functioning properly. The only time this works is if there are special conditions that make this possible, or if it's a modded bios specifically designed to be used that way. For example, I flashed my 9800p with a modded xt bios, and it works, but only because the bios was modded for 128mb of memory on my card, and also for the specific type of memory (there are like 3 different memory types used on the 9800). Also, the 9800p and the 9800xt are pretty similar cards in terms of the actual hardware. I don't know how similar the 9550 and the 9600 are, but unless you know that somebody has done this in the past I would strongly advise against flashing a 9550 with a 9600 bios.