Seeing as I work in retail and what not, I would probably say Hewlett Packard. They're nicely priced and generally have pretty good Tech Support. They also offer pretty decent extended warranties as well, if you don't feel like dealing with the retailer. As far as E-Machines and being capable of upgrading, I would have to disagree. Majority of the ones we carry only have PCI slots and their power supplies won't handle but one other hard drive and then won't handle an aftermarket videocard.
But, yeah, definately HP. Also see if a local shop can build the same, but I doubt they'll be able to beat the price. Manufactured computers are always cheaper! Especially with mail in rebates.