
First is the registry edit:
Step 1: go to run from your start menu and type "regedit"
Step 2: go to->
HKEY_LOCAL_MACHINE\SOFTWARE\PlayOnlineUS\SquareEnix\FinalFantasyXI
Change values for the keys '003' and '004' to hex values equivalent to the desired resolution in decimal.
For example, a resolution of 2048x2048 means you want both of the keys 003 and 004 to equal decimal value 2048, which is 800 in hex.
You can essentialy change your background resolution to anything you want. I run 2048x2048 (so use 800 for 003 and 004 keys) other resolutions you can try are :
1024x1024 (use 400 for keys 003 and 004) Even this settings makes a substantial difference and would probably do just fine on older/slower pc
1280x1280 (use 500 for keys 003 and 004)
1536x1536 (use 600 for keys 003 and 004)
1792x1792 (use 700 for keys 003 and 004)
2048x2048 (use 800 for keys 003 and 004)
now keep in mind this isn't going to change the resolution of your game this is just the background resolution so it doesn't have to match that of your monitor.
now as far as settings that I have within FFXI Config, I use this.
Screen size : Overlay graphics should be set to whatever your monitor resolution is. Background will now be completely blank due to the fact you forced a higher resolution through registry.
Textures tab : both set to uncompressed
Effects tab : Check enable bump mapping and set environmental animation to smooth.
Thats all, hope it helps