Decide what you would like to use this PC for, Ive just got my self a nice Sony VGX-XL100 media center PC. its dual core processor, has HDMI output for perfect large screen TV Hi Def action and its virtually silent, you can use it to record tv and play back any hi def stuff you wish, it will sit under your TV and blend in with your other AV equipment and has a wireless keyboard with touchpad. Has enough power to play decent games.
Apple mac's are nice, Ive owned a few but the lack of software really spoils it for me. if you like to tinker with things then you'll never find a wider range of software than there is for a Windows. As long as you take care what you download and click on Win XP should be just as stable as mac os or linux. Ive had a windows server running for over 300 days so far without so much as a flicker.
Really depends on what you want to use it for tho :) There's alot of choice for £800.00