Get a dual-core Acer desktop for $350 shipped
Buy.com has this well-equipped desktop, which is new, not a refurb, on sale for $349.99 with free shipping.
In my experience, most computer users have pretty basic needs. Word processing, Web browsing, e-mail, maybe a little money management and photo organizing. The Acer Aspire Slim ASL100-ED420A has more than enough horsepower for tasks like those, yet it's priced at a mere $349.99. Free shipping, no rebates, and it's not even a refurb.
The Acer's specs include an AMD 2.2GHz 4200+ dual-core processor, a 320GB hard drive, a DVD burner, and even an 802.11b/g wireless card (fairly rare in a desktop)--all packed into a slim, stylish case. The only shortcoming I can see is the 1GB of RAM, some of which is allocated to the GeForce 6150 graphics card. Because Vista (in this case Home Premium) all but demands 1GB for decent performance, this machine could prove a little pokey. If it does, bump the RAM to 2GB and you'll be golden.
Acer provides a 1-year warranty on parts and labor. This looks like a very solid desktop deal for students, seniors, the home-office crowd, or anyone else looking for, well, a solid desktop.