I have a 3½ year old Dell Dimension 4100, 933 mhz Pentium 3 and would like to upgrade to a 2.4 ghz Pentium 4 cpu or higher. The current mother board is an Intel D8155EEA AAA10383-404. Is this possible? Or do I have to replace both the motherboard and the cpu? What specific motherboard and cpu would I need? Would the present cooling system be sufficient? What else do I need to know?

Yes without doubt you need another mobo and processor. Best thing if you're not sure is to go buy a bundle deal including the whole lot. www.novatech.co.uk do some good ones if you're in the UK. The heatsink and fan are so cheap it's not worth keeping your old ones even tho it's the wrong shape!! :)

There are a couple of other issues that you will encounter. The Dell power supply uses a different plug connector than a normal atx power supply, and is also only rated at 200watts. The case switch connectors are proprietary too. In short you would be better off to keep your Dell, and just build a new computer using the best components that you can afford.