It shows there is a market for computers without monitors.
Otherwise, I don't know enough about computer tech to comment on whose hard drive is faster, etc.
ATA is an ancient standard for connecting the internal hard drive to the computer. It started to get phased out over the last year or so in favor of Serial ATA (SATA), which is the latest and greatest standard. I wouldn't buy any computer these days that only used ATA. I think the Dimension 3000 is the only desktop that Dell ships with ATA -- the rest of the line (starting at $800) uses SATA.
Anyway, I was wrong, the Dell apparently does ship with an el-cheapo monitor. OTOH, to get the Dell up to the likely feature set of the Mac would probably cost as much as that monitor. Updgrading XP Home Edition to something a bit closer to the Mac OS's abilities (XP Pro) would almost buy you a new monitor in itself.
Something else to read carefully is the statement of warranty. On the really low priced Dells, if you look carefully, you'll see a 1 year tech support warranty (for telephone support only), and you get a 90 day warranty on the hardware. You can upgrade the warranty, but it will cost extra. This is only on the Dimension line.
Mark