aha, i've FINALLY figured this out.
the setting is where i originally thought it should be: tasks --> settings --> tv --> configure tv or monitor. i have my TV attached with an S-video cable, and i had chosen that setting, which gave me NO options for changing resolution. apparently if i tell media center that i have a TV running through a VGA cable, i get a full list of display resolutions that i can choose from, and THIS works perfectly.
very bizarre, but fixed nonetheless. thanks everyone for your suggestions.