A motherboard install is probably the most complicated changeout that you can perform on your machine, since you basically have to take everything out and put it back in. That said, if you're careful and check things before turning it on, its not that bad of a process. You will, however, probably need to reinstall Windows, unless you match motherboard models perfectly (unlikely with an OEM motherboard like in a Dell). Windows may start with a different motherboard, but my experience is that such a major change leads to a very flaky system.
As for selecting a motherboard, you need to know the pin count (socket type) of your existing processor, its frontside bus speed, and its core clock speed. To use your existing memory, you'll also need to note the type. With this information, you can browse selections from major motherboard manufacturers (I suggest Asus, Gigabyte, Bob may also have some names). Once you have a model in mind, you can go through pricewatch.com or similar to get a decent price. A new-ish model Pentium 4 or Althlon XP motherboard should run too much more than $100, depending on the included features.
You should be able to use all of your existing devices and expansion cards, provided you select a board with the same types of slots and memory configuration as the existing part. The only feature I would caution you about is the power supply connector; the last Dell that I performed a PSU swap on used a non-standard ATX connector pinout, and an additional connector (looked kinda like half of an AT connector?). I'm not sure how widespread the incompatibility is; just something to watch out for.
If you have any questions regarding the selection or the install, there's plenty of help here.