M28, fundamentally, is very similar to M18 as they are both derived from basically the same core parts. Where M18 is derived from ATI's R420 core, M28 is derived from R423 - both of these cores are architecturally, and performance configuration wise, the same but R420 is built for AGP graphics whilst R423 is built for PCI Express. Where the configurations differ, primarily, is in the number of pixel quads that are enabled - while R420 and R423 have 4 pixel quads (16 pipelines), M18 utilised 2 pixel quads (8 pipelines) whilst M28 has 3 pixel quads enabled.