Try F8 to get the boot prompt
Then fill in : "Graphics Mode"="1280x1024x32" and press enter
btw, fill in the line including the ""
Cheers
oh, and if it works, you can edit the /Library/Preferences/SystemConfiguration/com.apple.Boot.plist file and add this in the Kernel Flags string :
...
<key>Kernel Flags</key>
<string>"Graphics Mode"="1280x1024x32"</string>
...

