The Celeron D will probably be useful longer than the Pentium 4. It is later architecture so later software will operate on it and possibly not on the Pentium 4 (but we are talking a ways down the road). Generally, though, the Celeron chip set is slower, producing less heat, and is therefore mostly found in laptops, where heat dissipation is a big issue. A Celeron D is about 75% the speed of a Pentium D. I think you will be satisfied with either.
As for staying with Dell, or any other manufacturer, it is a shot in the dark these days. The market demands prices remain as low as possible, which is the driving force for manufacturers purchasing computer components. Computer manufacturers contract periodically to purchase these components, and (this is sort of exaggerated but makes the point) if one manufacturer has a component for a penny less than another, even though the other is 100% better in every aspect, they will go for the cheaper component.
Nevertheless, for what you would like to do, again, I think you would be satisfied with either.
I hope this helps.