You probably want to try passing video=gbefb:1024x768-15@72 to the kernel -- or something similar. That exact parameter, will set the resolution to 1024x768, 15-bit colour (16/24/32-bit colour was b0rked in X last time I checked), at a refresh rate of 72Hz.
label=working image=/vmlinux append="root=/dev/sda2 video=gbefb:1024x768-15@72"
I'm using arcboot : I tried with video=gbefb:1280x1024-15@72, and it works perfectly ! Thanks a lot ! Guillaume