Try grabbing a newer kernel source and see what happens. I know my ti4400 supports 1024x768 by vga=792, running on the Fedora 2.4.22 kernel, so building a 2.4.22 might enable this for you. (this is not using rivafb).
"..the triumph of evil is for good men to do nothing."
VIDIA video driver RPMs for Fedora
:: see yum repo at livna.org