Here's an easy way to figure out the correct resolution for your T61, or pretty much anything else:

hwinfo --framebuffer

In the output, you should see something like this:
Mode 0x036e: 1680x1050 (+6720), 24 bits

Take that mode for your vga= on your kernel line:
title Debian GNU/Linux, kernel 2.6.21-2-686
root (hd0,0)
kernel /boot/vmlinuz-2.6.21-2-686 root=/dev/sda1 ro vga=0x036e
initrd /boot/initrd.img-2.6.21-2-686
savedefault

then, when you reboot, it should come up with the correct resolution.