I too have bought a new TFT 20" ACER display and cannot connect at 1400x1050. I can only connect at 1280x1024. I have the same card as SCB in a Dell Dimension 2400
The Dell 2400 cannot handle a lot of video card without a replacement power supply.
Whatever your budget allows out of the ATI, Matrox, or nVidia PCI cards on www.zipzoomfly.com or www.newegg.com with 64 MB or 128 MB will work well. The more memory the better.
Just remember this is a budget PC with Intel i845GV, and Celeron 2.4 GHz. The current video is integrated Intel Extreme. It runs cool as is.
You must use a PCI card. There are still good ones left. If you upgrade to the PCI video, upgrade the memory as well. If you go over 128 MB, replace the power supply.