I've managed to flesh out the list of supported resolution modes:
The standard TV modes will support a resolution from 256x224 for NTSC to 640x448 and 256x256 for PAL to 640x512. I'm not sure what the last three modes are since they were never implemented in gsKit or anything I've found. I'm guessing they're used by the dvd player elf and I'm assuming they're 720x480i, 720x576i and 720x480p. The additional modes will help with rendering low-res modes for fceultra and snes9x as well as the snes's hi-res modes.
Interlaced modes explanation .
I'll be using NTSC interlaced modes as an example. The NTSCi30 and NTSCi60 modes are for when reading the frame buffer using either frame mode or field mode. Field mode reads half the lines of a 448 line frame buffer, first the odd fields then even fields, and so displays a full image very 2/60th seconds or half the image every ~1/30th a second. Frame mode reads all the lines of a 224 line frame buffer and upscales it to 448 lines by essentially displaying the image twice on even/odd lines. The whole image is displayed every 1/60th of a second, even though it takes the same amount of time to stretch it as it does to display an entire 448 line image, 1/30th of a second.
Now on to fontm .