ATI's Radeon X550 chip doesn't appear on the company's website, but that hasn't stopped Taiwanese vendor Info-Tek announcing a pair of graphics cards based on the part....

The driver reveals the X550 is essentially a higher-clocked X300.
It's based on the same core as the older chip, the RV370, with four pixel pipelines and a pair of vertex engines. While the X300 is clocked at 325MHz, the X550 comes in at 400MHz

http://www.theregister.co.uk/2005/06/16/ati_x550/