I agree with everything said so far about the pros and cons of Dell, especially the bad news about proprietary parts. What stands out with them - and it shows in their market share - is their consistency and service. Of the name brand machines that have been out there for a decade or more, Dell's lows have been higher than anyone else's and their customer service, although difficult to understand these days from "Mike" in Bangalore, is excellent once the problem is identified. In general, if you buy a PC as an appliance and you just want it to run and be fixed quickly when it doesn't, Dell is an excellent choice - and I've never owned one, but I've purchased about fifteen for our workplace and they've been pretty dependable.
With other brands, variation in build quality and reliability is astounding. My first computer was a Micron when they were getting rave reviews for performance and reliability. I had a Pentium 66 that baked itself to death in 18 months, but three coworkers who bought Microns on my advice loved them. Then my wife, who couldn't tell a CD tray from a cupholder, said, "CompUSA has IBM's on sale. They're supposed to be good. Why not get one of them?" I did and it ran extremely well through two heavy teenage users. The IBM was replaced by a Compaq returned to Sears after Christmas. At that time Compaq had dreadful ratings, but it's had 4.5 bulletproof, easily- and often-upgraded years. So has a second Compaq my son bought at a Compaq auction - and great support. My latest, an HP MCE, has run great after I ditched the OEM mouse and went with a USB optical.
My son and I built a PC from leftover parts from the Micron and IT ran well. Yet he has had nothing but trouble with a supposedly gold standard IBM ThinkPad he bought for college (pre-Lenovo).
I would strongly consider eMachines for the reasons discussed in this forum plus one more - service and support. Especially as a second machine, when downtime is slightly less critical. I bought three eMachine refurbs for our business from the reseller that everyone loves to bust on. One has been flawless and the other two needed to have some parts replaced. There were three incidents and I called three times during normal business hours. My hold times were 15, 28, and 15 SECONDS, each rep spoke American, and they listened to my diagnosis instead of working from The Universal Diagnostic Checklist.
In general, the more customized the machine, the more easily it can be modified. The more store bought it is, the better the service, but the less room for expansion and tinkering you'll have.
After all of this, my daughter is going off to college in September. Based on all of our experiences, she'll be toting an Apple iBook.