Changing screen resolution

I installed CentOS 4.1 with Graphical Interface On – starting up using Gnome. I modified the screen resolution to 1280 x 1024. All worked well.

I just moved my server to a new location which has a standard 15′ VDU (which doesn’t accept 1280 x 1024 reolution). When I boot up, the screen goes blank. I had to change the runlevel back to 3 to get the system to boot in text mode.

Can anyone please tell me how I can get into the system (short of carrying in my 21′ VDU) to run X-configuration in 800 x 600?

You insight and direction would be greatly appreciated. Thank you.