After looking long and hard and not finding any 800x600 UIs out there, I decided on trying my own. Uses default images and such and each window can stand alone for mix and match ease. Just unzip all files into your UI folder (i.e. sita800x600) and you should be ready to go

Changed windows are:
By me: BigBank
Selector
Target
HotButtons
Inventory
By others (don't remember who, so if you recognize your work I thank you) :
Buff and SDBuff windows
Group window (slight modify by me, moved LFG button)
Compass
Player window
Merchant window (not shown in picture, shows everything for sale)

Hope someone gets some use out of it

11/5/04: don't know why i didn't post this sooner, but as a warning the buff/song windows aren't OoW compatible

4/28/05: The inventory window should now have the crystals that you get from DoN. Still no update for the buff/song windows yet.