I still can't get it why it didn't worked by editing the boot.plist and why you used 1440x900x60 where "x60" should have have been "x32" as 32bit color is the maximum color depth 60bit color depth does not exists yet??!!! Instead you should have used: <string>1440x900x32</string>.
You have to make sure to copy the com.apple.Boot.plist on your desktop then edit the copied one and replace it in Extra folder, as you can't overwrite the original one, it wont let you save it.
Still if it doesn't work, you can download and install the Chameleon boot loader, it will let you choose the resolution and some more options.