I'd definitely try matching your overlay with background res so it's only drawing exactly what you're seeing. It actually gives it a nice effect, though doubling it the way Hypnotizd earlier showed increases the effect and makes it even nicer generally. My laptop can only managed 1:1 though so maybe you're in a similar situation. All this can be done via
http://www.ffxicalculator.com/ and very simply!
Aside from that, I use this program in both Vista and Windows 7 for XI:
http://www.snapfiles.com/get/vsoptimizer.html - It
seems like one of those "It fixes your computer!" BS programs, but it actually works really well and buys me quite a few FPS when playing and allows me to run Firefox, XI and a media player at once. As a note, you don't have to use it for its original purpose and go through configuring stuff, just turn on game mode before you play and you're good to go!
As a last note, try disabling sound in the FFXI Calculator "FFXI Settings" tab and reducing the maximum amount of characters on screen (in-game setting) to see if it changes anything. Sound makes a huge difference for me.