Just configured a new Dell machine with as-close-as-i-could-get specs as the base £799 iMac.

Dell
Intel Core Duo 2 2.0Ghz
19” Monitor (It’s not 20” but closest I could get)
Vista Ultimate
320GB drive (That’s bigger than the iMac)
16x DVD +/- RW Drive (Again faster)
256 ATI Graphics (Bigger than the iMac)
Dell Enhanced Wired Keyboard
Dell Optical Scroll Premium Mouse
Speakers
Webcam
Core Snapfire (iLife is free remember)
No Bluetooth (Couldn’t see an option for it)

Total: £695.74

iMac
2.0GHz Intel Core 2 Duo
20” Glossy Display
250GB Drive
SuperDrive 8x (DVD±R DL/DVD±RW/CD-RW)
New Apple Keyboard
Mighty Mouse
(Speakers, WiFi, Bluetooth, FireWire, Apple Remote and iSight are built in, not options like Dell)

Total: £799.00

So a like-for-like dell will save you £100, and you’ll get better graphics and a larger hard drive. But, spend that extra £100 on an iMac and get iLife, a remote to use Front Row, Bluetooth and something called Mac OS X Tiger.

More to the point, what sort of machine do you want to look at every day?

I’d say that’s worth an extra £100.