How to change Radeon driver screen resolution before boot

Hi,
I'm relatively new to Linux so I don't know a lot about configurations, commands, etc. I chose to install and dual-boot Kali Linux and my Windows 7 because I'm learning both python for penetration testers and some Linux books. Like 2 weeks ago I managed to install Kali Linux (first linux install ever) without any single problem, pretty easy in fact. I updated every package according to the official Kali Linux manual and everything went ok. Yesterday, seeing that Kali Linux was not using the driver for my GPU (horrible screen resolution and high CPU temperature/usage), I decided to install the official Radeon drivers following an online tutorial. Everything went totally ok, until I had to reboot the PC. Trying to get into Kali Linux (either normal or the recovery mod) would give me an "out of range" on my monitor. I read that this is because Linux defaults to 1440x1050 (or something like this) when my monitor only supports 1440x900.

I'd post links to similar threads, but I literally read at least 50 with the same-similar problems, but none of them has worked for me.

I'm able to enter my Windows 7 partition without any problem, and used a tool to enter the Linux partition.

Things I tried:
-Going to the GRUB edit menu, 1) changing "radeon.mode=0" to "=1", 2) deleting it, 3) deleting it and putting vga=773, 4) deleting it and putting vga=ask
-Backup-ing and editing /boot/grub/grub.cfg, changing the same as above
-Backup-ing and editing /boot/config-3.14-kali1-amd64, changing "CONFIG_DRM_RADEON=m" to 1)"=n" and 2)"=y"

Things I did not try (don't know if it would do anything):
-Change default desktop mode

Please excuse me if this is not the correct place to ask about this. I'm willing to try any suggestion you give me and if there is anything else I should provide you, please tell me.
Thanks for your time.