If I was going to buy a brand new machine right now, which I'm not cause I just upgraded most of my key components, here's what I would want.
First, Build for what you want to do with it. I am wanting to play some high end games and also use it for DVD and also for doing graphics and website stuff, so I have the need for a fast processor, lots of ram and a big video card.
If you use it for surfing and document work, then start with the motherboard, and get something that can be expanded, and add in a core2Duo processor. Get a hard drive as big as you can, so you can store digital photos, movies, music, etc. Then get as much ram as you can afford. flat monitors are really cheap. I prefer Samsung for both quality and price. Get a large one, always worth the investment if you ask me. These days you can get a good 20" plus monitor that can double as a second TV.
Most of the new Dells and boxed machines are really intended for this kind of user, so you are fine to get one premade as opposed to building yourself. With the state of the economy, you get a lot of bang for your buck. Have fun, do your shopping around.