I don't know if this is known (I didn't see it in any of the tutorials in the sticky thread), but there is a quick and effective way to get rid of jagged corners. We will use the famed RX8 Concept, which is very good at embarrasing the PS2 in most instances.
These are the windows and headlight of an RX8. All I did was polygon-select around the jagged edges and median between 1 and 3 pixels.
These pictures are at 100% size, so if you are doing a 50% reduce like I do, they will be all gone.


These are the windows and headlight of an RX8. All I did was polygon-select around the jagged edges and median between 1 and 3 pixels.


These pictures are at 100% size, so if you are doing a 50% reduce like I do, they will be all gone.