There are several guides on the internet that detail how to build a computer. A little googling should yield a good one.
Most motherboards these days are going to be DDR2. Unless you buy a used one, going with DDR2 would be easier, and it probably won't cost you that much extra money.
Newegg.com always has some good deals on
CPU/motherboard combos. I'd check there if you are planning on buying new. You will NOT find a P4 motherboard for a decent price. It's too obsolete, and anything you do find will likely not be very capable and will cost far more than newer ones will. I found this out several years ago when my P4 board fried.
Get a decent AMD processor in the $50-100 range; it will serve you well. I bought an AMD 64 3800+ two years ago for $55 or so, and with graphics cards upgrades, I can still play most modern games.
If you get a new motherboard/cpu, it's not likely your Dell power supply will be compatible, particularly if the new board has an AMD processor. Also, I've learned the hard way that it's not a good idea to skimp on power supplies. While
some cheap power supplies are ok, the vast majority are not to be trusted. I had to rebuild twice due to cheap power supplies - a crappy power supply can fry your motherboard or destroy your video card. Another problem with cheap power supplies is that their true rated power doesn't come close to what the manufacturers claim it to be.
Seasonic psu's are ok. Corsair makes some of the best ones for the price, though, however they are a bit spendy (my 450 watt Corsair was in the $80 range when I bought it a year or two ago). Newegg currently sells a
Corsair 400 watt model for $50 that would probably work extremely well for your purpose.
Incidentally, Corsair also makes some really good memory, as well, and their value DDR2 RAM is usually very well priced.
Like Zahc said, if the CD drive works, it works. You may want to look into something that can play and burn DVDs and CDs, though, if your current model can't. They're more convenient.