If you want to movie and photo editing and would like a little more speed, then a G5 would be your best bet. You can get good deals in the Refurb section of the Apple store. And there's always eBay.
John Lewis aren't doing a bad deal - low end PowerMac 1.8 for £999, and it also comes with a 2yr guarantee.
And whatever you buy, don't forget that Mac's have a great re-sale price. So if you don't like it you'll get some money back.