vbetest
Install the lrmi package from the AUR that contains the vbetest tool (x86_64 users will need to use #hwinfo above).
Run vbetest as root
Then note the number in [ ] corresponding to your desired resolution.
Press 'q' to quit vbetest interactive prompt.
As an option, in a console as root, you can test the mode you just picked up by running vbetest -m <yourcode> and see a pattern like this one
Add 512 to the discovered value picked up above and use the total value to define the vga= parameter in the kernel options of menu.lst.
Reboot to enjoy the result
For example vbetest on one computer:
[356] 1440x900 (256 color palette)
[357] 1440x900 (8:8:
So here the number you want is 357. Then, 357 + 512 = 869, so you will use vga=869.