I did a clean upgrade on an old machine I've got at home from RH8 to Fedora Core 1 last night. Under RH8, the machine ran 1024 x 768 no problems at all despite the ageing Voodoo 3 3000 and an 15" unspecified monitor. But the display in Fedora Core 1, won't allow me to increase it past 800 x 600 at unspecified.

When I change the setting to say that it is a generic 1024 x 768 monitor and I reboot X. I completely lose the display, and then have to rebuild Fedora to get it back again.

Does anyone know a way in which I can get it to work at 1024? 800 x 600 really grates after a while, I'm just glad its not my main box.

well there must be a name / model written on the monitor somewheres ...
and when you find that information out, then do a research on google, or a search engine of your choice and find information about your monitor mainly your horizontal and vertical frequencies ... i namely suggest your monitor cause you stated you have a V3 3000 and i know that is in the list of choices when configuring X...
so when you do find your monitors specs ...
go into the terminal and as root run
xf86config
and manually configure your monitor and pick your V3 from the list, and try again ...

I had another hunt round the monitor and found these markings:

XIOD 60/50Hz
P/n CXK-15E
S/n FK9207542

Which haven't yielded anything useful, when searching the net for them. Any further ideas?

Try adding the resolution to XF86Config-4 - put it before "800x600" (ie "1024x768" "800x600 "640x480"), restart X and it should work.