Sorry mods for all the pictures.
Also, step by step directions w/ pictures for XP users:
Step 1: Click Start and go to Run
Step 2: Type in "regedit" (no quotes) into the box that pops up.
Step 3: You will now be in your Registry Edit screen. Follow the menu as the screenshots show:
Step 4: (Double click on "Final Fantasy XI" after "SquareEnix") You should now be here:
Highlight 003 as shown.
Step 5: Right click on 003, select Modify.
Step 6: Modify the registry data. Be sure that Hexidecimal is checked. Default should be 200. The highest you can go is 800. You can set them in incriments of 100. (200, 300, 400 ect.) This will make the game think you're on a 2048/2048 resolution. Chances are, you're running on a smaller resolution, so what this will do is shrink it down to your resolution, making the images appear much cleaner and smoother. For example, my resolution is 1600x900, obviously smaller than the 2048x2048. Think of it as resizing a picture. The bigger you make it, the crappier it looks. The smaller you make it (which is what we're doing here), it'll look either the same or better. In the case of FFXI, it looks a lot better.
DO NOT GO PAST 800 OTHERWISE YOU WILL GET AN ERROR AND POSSIBLY CRASH!
Step 7: Repeat Steps 5 and 6, only this time on 004. Make sure this value is the same as the one you set for 003.