The advantage over the iMac's is that the G4 range of Apples allows you to add additional cheap internal hard drives for storing video material. If the iMac monitor is too small for you then the G4 allows hooking up to a larger monitor making long hours spent editing easier on the eyes . They have a quicker processor than the iMac so if you see yourself doing a lot of rendering of effects then a G4 is more suited.
Models include 733MHz, 667MHz, 533MHz and 466MHz single-processor configurations, and the dual processor 533MHz Power Mac G4 (available from the Apple Store). And if you pick a model with a built-in DVD-R/CD-RW SuperDrive, you not only get iMovie video editing software (pre-loaded on every new Power Mac G4), you also get Apple's revolutionary new iDVD software -- the easiest way ever to turn Desktop

Prices vary according to your spec. Go to the Apple web site to learn more about this.
Top of the range: (monitor not included)
£2,499.00       733MHz PowerPC G4
256K L2 & 1MB L3
256MB SDRAM 60GB
Ultra ATA  DVD-R/CD-RW drive
NVIDIA GeForce2 MX    Gigabit Ethernet      56K internal modem

£1,199.00 466MHz PowerPC G4
1MB L2 cache
128MB SDRAM
30GB Ultra ATA
CD-RW drive
ATI RAGE 128 Pro
Gigabit Ethernet
56K internal modem

The iBook Portable Laptop
The iBook is fine for editing. Increase the memory size though to at least 192mb and of course you will certainly need to buy an external firewire drive for storing the video material.
The iBook lets you make your own movies -- and show them anywhere. It lets you create your own music CDs and provides CD music conversion to MP3 files. It makes a dandy photo album, with space to spare for the thousands of images you've captured with your digital camera. And it gets you on the Internet in record time -- wirelessly, if you prefer. What's more, with up to five hours of play-time provided by its lithium-ion battery, chances are your iBook won't need recharging before you do.
Starting price £915 plus VAT