Thanks for the advice Lorand. It still didn't work so I tried another monitor (Philips) and it's the same thing! Can't get more than 16bit 640x480. Tried downloading the Philips driver too, (both monitors display can be adjusted when using other computers)
Again, I'm not too computer savvy but its an Optiplex GX110, I upgraded the bios to 09 if that matters. 98SE.