You can use vbetest to see what VESA graphics modes your video card is capable of. Install it (yum install vbetest), then just run the command vbetest. You should see a list of resolutions, with the mode in square brackets. Add 512 to the number in brackets to get the number for vga=<number>.
If 1280x800 does not appear in that list, or if the listed modes don't work, then you will probably have to use the radeonfb module, or compile it into your kernel. In the latter case, you would use it as a kernel option in grub. For example, for my Radeon I use a 1024x768 console framebuffer at 76Hz in 32-bit color like this:
kernel /vmlinuz-2.6.27 ro root=/dev/sda1 video=radeonfb:1024x768-32@76
If you use radeonfb as a module, then you'd have to use the fbset command (part of the fbset package). It may take some experimenting to get the right combination of resolution, vertical frequency and color depth.