I like what Fed said BUT I just purchased (for a family member) an Acer in a nice, quiet case with a Sempron 3100 processor, standard Gigabyte motherboard (even tweakable!), 512 RAM, onboard video but an 8X AGP slot, 160 gig SATA hard drive, DVD burner flash media slots, keyboard and mouse, 1 year warranty and XP Home for <$300. I doubt if you could build one for that price, and certainly not have it assembled. Of course that's not state of the art, but it is quick, full featured, nice looking and non-proprietary (and I didn't have to put it together for him!).
Every Dell Optiplex I ever got had on site 3 year warranty, bumper to bumper but that's been 3+ years since my last one. They are somewhat proprietary, but replacement parts are readily available. Sony is a distant follower in my book, right behind Compaq.
By the way IBM should be removed and replaced with Lenovo. IBM home computers are a thing of the past.